If President Reagan wants one thing from his plan to overhaul the tax system, it is to simplify the average American's daunting chore of filling out a federal income-tax return.

At his news conference last week, Reagan observed that he had to hire a tax lawyer to complete his return and said, "When I finally saw it, it was all made out, and I still didn't understand it."

Today, the Senate Finance Committee is to consider individual income taxes. If the panel passes the proposal on its agenda, Reagan may be even more baffled by his next tax return.

Under the plan drafted by committee Chairman Bob Packwood (R-Ore.), all Americans could take tax deductions for home-mortgage interest, but only some would be allowed to deduct car-loan interest. All could write off state and local income taxes, but none could deduct sales or personal-property taxes.

In the top bracket, proposed to begin at $35,000 for single taxpayers, some deductions could be written off in full while others would be partially subject to taxes.

The former would be home-mortgage interest, charitable contributions and real property taxes; the latter would include medical expenses, state and local income taxes, union dues and even tax-preparation fees.

"This is very clearly more complicated than the current system," said Stephen Corrick, a tax partner at the accounting firm of Arthur Andersen & Co. "It will create confusion, anxiety and ultimately mistakes. What we're looking at here is the accountant's retirement act."

The plan to make some deductions more beneficial than others is a significant feature of Packwood's tax-overhaul package and would bring in $21.1 billion by 1991. Its defeat would put the panel $50 billion in the hole as it tries to draft a tax bill that raises as much money as the current tax system.

Numerous senators, led by Sen. Daniel Patrick Moynihan (D-N.Y.), have attacked the plan to limit state and local income-tax deductions in the top brackets.

Moynihan calls it "a genuine invasion by the federal government of the autonomy of state governments." Sen. David H. Pryor (D-Ark.) calls it "a tax on part of a tax."

Packwood emphasized that he had left untouched some of the most widely used deductions, such as real property taxes, which he called "the backbone of most local governments." Moreover, only about 10 percent of all taxpayers would fall in the top tax bracket, the one in which benefits from certain deductions would be limited.

"I don't think it's a good idea, but I think it will survive because it saves a portion of some deductions that would otherwise have to be eliminated. It's a compromise position," Sen. Charles E. Grassley (R-Iowa) said.

But compromises tend to be complicated, and this one is no exception. Under the Packwood plan, taxpayers in the 35 percent bracket would be able to take most itemized deductions only against the 25 percent rate. That means, for example, that deductions for $100 in medical expenses would reduce their taxes $25 rather than $35.

At the same time, home mortgage interest, charitable contributions and real property taxes could be deducted against the 35 percent rate, requiring each taxpayer to perform two sets of computations.

The result is that upper-income taxpayers would get no greater benefit from deductions than those in the 25 percent bracket. Some tax policy experts praise this approach as a move toward equity, but they acknowledge that it conflicts with the president's goal of simplicity.

Take Reagan's tax return: The Packwood plan would give the president the full 35 percent benefit of his $7,500 deduction for a charitable contribution to Eureka College. But Reagan's $33,000 deduction for investment and legal advice would get him only a 25 percent benefit and would have to be calculated separately.

The Packwood proposal, like the House bill passed last year, has numerous other complicating features for individuals -- all the result of efforts to broaden the tax base so that rates can be lowered.

Only $1,000 of consumer interest, on credit cards and on loans for cars and other purchases, could be deducted under Packwood's plan. Married couples filing jointly could deduct $2,000. The current law has no such limits.

In addition, the $2,000 personal exemption would be adjusted, depending on the tax bracket. A family of four in the 35 percent bracket would get $8,000 in exemptions. The family's tax bill would decline, not by $2,800 (35 percent of $8,000), but by $2,000 (25 percent of $8,000).

Moreover, the exemption would be phased out for taxpayers earning above $100,000. Emil Sunley, a tax expert with the accounting firm of Deloitte Haskins & Sells, calculates that the phase-out, combined with the 25 percent restriction on the exemption, would give the family an effective, marginal tax bracket of 37.5 percent. Reagan has insisted on a top rate of 35 percent.

The promise of simplicity remains firm only at the lowest income levels. Packwood proposes to raise the standard deduction and personal exemption high enough to remove millions of low-income Americans from the tax rolls altogether. However, those changes will be possible only if the committee raises money by tightening loopholes and exemptions.