The day may not be too far off when District of Columbia taxpayers can use most of the same figures to make out their federal and D.C. income tax forms.

Mayor Walter E. Washington asked the City Council this week to bring the city income tax law into closer conformity with the federal code - a change that would slightly reduce the city's tax take and greatly reduce the aggravation taxpayers feel when making out the forms.

Marion Barry (D-At Large), chairman of the City Council Finance and Revenue Committee and a rival to Washington in the current mayoral campaign, said the committee would consider the proposal tax law change as soon as possible.

In a letter to the council, the mayor said his proposal would carry out recommendations made last December by the D.C. Tax Revision Commission, a citizens group which proposed a wide range of reforms in the city's tax structure.

If the council accepts the mayor's proposal, taxpayers would be able to take advantage of the changes when they file their returns in the spring of 1980.

Under present laws, D.C. taxpayers often find filing their local returns to be a nightmare. Allowable deductions and legal definitions used for the city returns vary greatly from those used for the federal tax form, so most taxpayers have to develop two or more sets of figures.

By contrast, many states - including Maryland and Virginia - have adjusted their own tax laws so that taxpayers may simply copy most income and deduction figures from the federal forms onto the state forms, and then use state tax tables to calculate the dollar amount of state tax that is owed.

That is substantially what the mayor is proposing for District taxes. The city would continue to control the actual rate of the tax levy.

The problem with the D.C. taxes arose because the city tax law, enacted in 1939, was not amended through the years as the federal tax law was changed.

Under the mayor's proposal, the city's yield from the income tax would drop by $2.5 million a year, a minor fraction of more than $180 million the income tax produces.

Also pending before the council is a more far-reaching change in the tax system sponsored by Hilda Mason (Statehood-At Large). Her proposal provides for total conformity between federal and D.C. tax laws, which would allow the taxpayer to make out only one form and would permit the Internal Revenue Service to collect city taxes at no cost to the city.