<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-02-18</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000352541</issuerCik>
        <issuerName>ALLIANT ENERGY CORP</issuerName>
        <issuerTradingSymbol>LNT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001209101</rptOwnerCik>
            <rptOwnerName>HARVEY WILLIAM D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>PO BOX 2568</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MADISON</rptOwnerCity>
            <rptOwnerState>WI</rptOwnerState>
            <rptOwnerZipCode>53701</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>PRESIDENT &amp; COO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON IN TRUST</value>
            </securityTitle>
            <transactionDate>
                <value>2005-02-18</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>58.243</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>27.1401</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>6081.9549</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>401(k)</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14832.3769</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>COMMON IN IRA</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON (RESTRICTED)</value>
            </securityTitle>
            <transactionDate>
                <value>2005-02-18</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>139.2279</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>27.1401</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>14536.0193</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON IN SHADE TREE PARTNERSHIP</value>
            </securityTitle>
            <transactionDate>
                <value>2005-02-18</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>27.0496</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>27.1401</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>2825.6254</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>BY PARTNERSHIP</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The reporting person acquired 58.2430 shares under the company's dividend reinvestment plan, pursuant to a dividend reinvestment transaction exempt from Section 16 under Rule 16a-11.</footnote>
        <footnote id="F2">The reporting person acquired 139.2279 shares under the company's dividend reinvestment plan, pursuant to a dividend reinvestment transaction exempt from Section 16 under Rule 16a-11.</footnote>
        <footnote id="F3">The reporting person acquired 27.0496 shares under the company's dividend reinvestment plan, pursuant to a dividend reinvestment transaction exempt from Section 16 under Rule 16a-11.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>F. J. Buri as POA for</signatureName>
        <signatureDate>2005-02-22</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0202
4
2005-02-18
0
0000352541
ALLIANT ENERGY CORP
LNT
0001209101
HARVEY WILLIAM D
PO BOX 2568
MADISON
WI
53701
1
1
0
0
PRESIDENT & COO
COMMON IN TRUST
2005-02-18
4
J
0
58.243
27.1401
A
6081.9549
D
401(k)
14832.3769
D
COMMON IN IRA
1000
D
COMMON (RESTRICTED)
2005-02-18
4
J
0
139.2279
27.1401
A
14536.0193
D
COMMON IN SHADE TREE PARTNERSHIP
2005-02-18
4
J
0
27.0496
27.1401
A
2825.6254
I
BY PARTNERSHIP
The reporting person acquired 58.2430 shares under the company's dividend reinvestment plan, pursuant to a dividend reinvestment transaction exempt from Section 16 under Rule 16a-11.
The reporting person acquired 139.2279 shares under the company's dividend reinvestment plan, pursuant to a dividend reinvestment transaction exempt from Section 16 under Rule 16a-11.
The reporting person acquired 27.0496 shares under the company's dividend reinvestment plan, pursuant to a dividend reinvestment transaction exempt from Section 16 under Rule 16a-11.
F. J. Buri as POA for
2005-02-22