The first year of tax overhaul would bring an unpleasant surprise to many people anticipating hefty decreases in their tax bills.

The tax-revision bill approved last week by the Senate Finance Committee, as well as the one passed by the House last year, would repeal many popular deductions on the first day the law takes effect, but would not cut tax rates for individuals and corporations until six months later. The standard deduction would increase a year after the law is implemented.

The House bill would reduce income taxes an average of 8.4 percent for individuals while the Senate bill would lower them an average of 6.3 percent. But those figures apply to the second year of the bills. Tax experts say those figures would be smaller the first year any bill is in effect.

"We are in 1987 going to have the situation where you have a broader tax base and a higher rate" than the fully phased-in rates of 15 percent and 27 percent of the Senate proposal, said Gillian Spooner of the accounting firm Touche Ross.

Both measures would bring in more money during their first year of implementation than will be raised if the tax code is not changed, although over five years both would be virtually revenue-neutral.

The House bill would increase revenues by $7.3 billion in the first year. The Finance Committee bill, analysts estimate, would raise more than that, but just how much more is hotly debated.

Neither the Joint Committee on Taxation nor the Treasury Department, the only two entities with computer models capable of estimating the effects of such massive changes in the tax system, has done a comprehensive estimate of the year-by-year revenue changes from the Senate bill.

A senior administration official, presenting the results of a Council of Economic Advisers' analysis of the legislation on the condition that he not be identified, told one group of reporters Tuesday that it would raise $22 billion in the first year. To another group of reporters, however, he declined to give a figure.

Yesterday, a spokesman for the official said he had stopped using the $22 billion figure after Finance Committee and Joint Taxation Committee staffers told reporters that no such revenue estimate had been produced.

Revenue estimates will not be available until the provisions of the legislation are published. Although the Finance Committee approved its bill on May 7, the staff is still working out many details.

"In past times some of the drafting has not been done. Now it would seem some of the crafting has not been done," said Ira Shapiro, director of tax policy for the Washington office of the accounting firm Coopers & Lybrand.

Meanwhile, Sen. David H. Pryor (D-Ark.) yesterday urged Finance Committee Chairman Bob Packwood (R-Ore.) to delete from the committee bill a provision awarding about $500 million in special tax benefits to ailing steel companies.

Pryor called the provision "a complete contradiction of the wishes of the majority of the committee," adding that he did not know of it until it was reported yesterday in The Washington Post. Congressional aides said several other Finance Committee members were not aware of the provision when the panel voted unanimously to pass the bill.

The steel provision is one of many as yet unpublished "transition rules," or temporary exceptions from the tax legislation, privately approved by Packwood for particular industries or groups. "If this loophole stays in the bill," Pryor said, "we're going to see one amendment after another offered to give similar breaks to other companies."

Finance Committee aides continued their policy of not commenting on the transition rules, saying that they will be made public after they are drafted.

The House bill would be effective Jan. 1, 1986; the Senate measure a year later. Rate cuts would begin this July under the House bill and the next July under the Senate bill. Instead of getting a tax cut halfway through the year, taxpayers would pay at "blended" rates that incorporate half the cuts for the entire year.

But taxpayers would lose some deductions the day the law takes effect. The Finance Committee bill would repeal immediately deductions for: state and local sales taxes, two-earner couples, deposits into tax-deferred Individual Retirement Accounts (unless the taxpayer is not covered by a company pension plan) and interest on nonmortgage loans. The interest-deduction repeal would be phased in over five years.

The Senate bill would also place limitations on the personal exemption and on use of the bottom rate of 15 percent for upper-income taxpayers.

Senate Majority Leader Robert J. Dole (R-Kan.) said yesterday he will attempt to change those limitations when the measure is considered by the full Senate next month, on the grounds that they impose a hidden, higher tax rate on upper-income taxpayers.

Low tax rates for capital gains -- the top rate now is 20 percent, compared to a top rate on salary income of 50 percent -- also would be eliminated.

Those low rates are achieved by excluding 60 percent of capital gains from taxation and by taxing the remainder of those gains at regular rates.

Because repealing the exclusion immediately would result in a top capital gains rate of 38.5 percent in the first year under "blended" rates, far higher than the current top rate, Finance Committee aides said the bill will include a transition provision setting the top rate for capital gains at 27 percent.

Corporations also would be in for a shock during the first year of a bill.

The Senate plan would repeal the investment tax credit, a subsidy for equipment purchases, as of Jan. 1 of this year even though the rest of the bill would take effect in 1987.

And the Senate proposal would limit the use of existing investment credits to 70 percent of their value, although that limitation may be phased in along with the rate reduction for corporations, congressional aides said.