The Internal Revenue Service is changing its income tax forms for 1985 in ways it hopes will reduce taxpayer cheating, IRS officials said yesterday.

Promising to do a better job in processing returns and mailing refunds, IRS officials admitted that the additions to the 1985 tax returns move away from tax simplification, a popular notion these days. But they pointed out that most of the changes were ordered by Congress, and that they are not as extensive as revisions to tax forms in previous years.

The additions to the forms in part reflect the changing shape of society: Several have to do with divorce and custody of children.

For example, ex-spouses who pay alimony now will be required to report the name and Social Security number of the person who receives the payments, to make sure the recipient declares the money as income. And parents whose children do not live with them will be required to report the childrens' names to make sure that more than one parent does not claim the dependents' exemption.

"Now we have the ability to check both the deductions and the reporting of income to see if both parts match," said Art Altman, assistant director for tax forms and publications.

Additional tax-compliance changes will require more extensive record-keeping for business expenses and real estate depreciation and require those with Social Security income (which is partially taxable) to report income received from tax-exempt bonds.

Other changes include an expansion of the deduction for charitable contributions for nonitemizers from 25 percent in 1984 to 50 percent.

IRS officials said that they are taking numerous measures to ensure that the filing season goes more smoothly than it did last year, when late refunds and service errors reached record highs.

"I'm very hopeful that it will be a substantially different year, in that it will be much improved," said Fred Perdue, director of returns processing.

The IRS has $31 million more to work with than the roughly $3.6 billion budget request for 1986 made by the Reagan administration, and officials said that money is being used "across the board" to increase staffing, reduce employe workloads and add computer capacity.

But President Reagan earlier vetoed an appropriations bill that would have given the IRS an increase of more than $170 million instead, and the $31 million is part of an omnibus funding bill that is subject to change by Congress.

That $31 million is "absolutely not enough," said George King, spokesman for the National Treasury Employees Union.