President Reagan is considering proposing in his State of the Union address a major restructuring of the federal income tax, perhaps just to simplify it, perhaps to convert it from a tax on income into one on consumption.

That way there would be a tax on income spent, but not on income saved.

In his first year in office the president stressed cutting taxes and pushed a big three-year tax cut through Congress. Last year he shifted, agreeing to two tax increases that in effect partly repealed the cut, and he and his advisers are discussing further tax increases now. The new proposals would be a third phase of policy, a kind of tax "reform."

Treasury Secretary Donald T. Regan gave intimations yesterday of the new tax position at a Washington Post luncheon. "We have a tax code in this country that has more than 1,000 pages," he said in a speech. "It is only the well-to-do who can afford the lawyers and accountants needed to really pore over the whole matrix."

Although the president seems likely to propose tax reform in the State of the Union this month, no one thinks Congress will act on the issue soon, and some officials fear the proposals will be dismissed as no more than an effort to divert public attention from a fiscal 1984 deficit and other bad budgetary news.

Regan said the "president has not given me his final decision in the tax field," but, according to various sources, the president is considering two basic alternatives.

The first would follow lines suggested by many tax reformers--elimination of most deductions, exemptions and credits. The result would be a major broadening of the tax base, which, in turn, would allow a major reduction in tax rates from the current levels of 14 to 50 percent without major loss of income.

Basic issues to be resolved in this approach include whether the tax system's progressiveness should be retained or modified, and whether reform of individual income tax should be accompanied by restructuring of the corporate income tax.

Although the notion of a "flat" tax--with only one rate--had initial administration support, many officials have backed off because it would almost inevitably shift the tax burden from the upper brackets onto the middle and lower middle classes.

The second alternative under consideration involves a far more radical change: a shift from an income tax to a consumption tax, a step that would likely lead to eliminating the corporate income tax.

In testimony last year to the Senate Finance Committee, Assistant Treasury Secretary John E. Chapoton outlined what he described as a "uniform tax on consumed income." It would work this way:

In calculating tax liabilities, an individual would first total up all wages and fringe benefits, dividends, interest income, proceeds from the sale of stock, property or other assets, and, perhaps most important, any borrowed money.

From this tax base, the individual could deduct all money put into savings; the cost of buying stocks, bonds, shares of mutual funds; the cost of business investments in new equipment and other assets; and repayment of loans and interest on loans. The balance would be taxable.

Advocates of a consumption-based tax say it encourages savings and investment, which, in turn, provide for economic rebuilding and expansion.

Chapoton contended that such a tax could be designed so that no income group would be severely penalized. Other critics have argued, however, that there is a strong bias against the poor and lower middle class when consumption is taxed, because they need to spend a much higher percentage of their income to meet basic requirements for food, clothing and shelter.

In addition, other critics have argued that the exemption for savings and investment would encourage the concentration of wealth among those who can afford to invest and who would gain most from sheltering investments.

Although Chapoton did not specifically endorse changing the tax system, he was enthusiastic about a uniform consumption tax. He contended that it would result in a higher savings and additional capital formation, would be simpler than even a reformed income tax and would encourage efficient investment.