President Carter is expected to propose next week that, for the first time, substantial amounts of income-tax revenue be used to help pay Social Security benefits.

In addition, the President is expected to recommend that Congress change the Social Security tax so that employers would have to pay it on all wages they pay an employee and not just on a portion of those wages, as now.

The so-called wage base - the amount of an individual's earnings on which the Social Security tax is levied - also would rise for employees, but not all the way, as it would for employers.

The wage base is $16,500 this year, and is scheduled to rise each year in the future by the same percentage as wages generally in the economy. Thus experts expect it will be $17,700 in 1978, $19,200 in 1979, and so on.

Carter would add to these scheduled increases an extra $600 in 1979 and $600 more in 1981, 1983 and 1985 for a total add-on of $2,400.

In the last several years, Social Security benefit payments have started to outrun Social Security tax collections, and the system's giant trust or reserve funds have been drawn down.

These reserve funds will be exhausted in the early or middle 1980s if Congress takes no action to reconstitute the system, which now pays benefits each month to more than 30 million individuals - roughly 1 American in 7.

Carter's expected recommendations are the way he wants the system shored up. But they are sure to run into resistance from employers and those who believe that only the Social Security tax should be used to pay benefits under the plan.

The Social Security tax is currently 5.85 per cent for employer and employee on the wage base. The tax rate is scheduled to rise to 6.05 per cent for company and worker next year. That increase was originally planned to help pay Medicare costs, which also come out of the Social Security tax. But the Medicare trust fund is in relatively good condition, and another step Carter will propose in his message, probably Monday, will be to divert half of this increase to pay for retirement and disability benefits.

President Ford proposed last year, and again in January before he left office, that Social Security's financial problem increase in the tax rate beyond this scheduled one.

But Carter said in last year's presidential campaign that he opposed that step on the grounds that such a rate increase would fall harder, in relative terms, on low-income workers.

Social Security amounts to about one-fifth of the federal budget. Benefit payments to the elderly, the disabled and survivors of workers who have died are expected to be $95 billion in the fiscal year beginning Oct. 1. Tax receipts in that period will be $88.9 billion.

There are several reasons for the system's financial problems.

The simultaneous high unemployment and high inflation of the last several years have hurt. With high unemployment, total wages drop, as do tax receipts based on those wages. Benefits, on the other hand, are pegged to prices, so that high inflation means big increases in the system's costs.

A second, longer-range problem is that the birth rate has declined after the baby boom of the 1940s and 50's. This will eventually mean a change in the ratio of workers to retirees; there will be fewer workers to support each elderly beneficiary, and each worker will have to pay higher taxes if nothing else is done.

Another problem is a mistake Congress made in 1972 when it voted to have benefits rise each year with prices. Congress over-adjusted for inflation, and another step Carter is expected to propose is a correction of this over-adjustment.

Carter's proposal to shore up the system partly with an income tax injection will be presented as a temporary expedient, to last until the next Social Security advisory committee can pass judgement on the idea. That would be a year or two away.

In the meantime, the Carter recommendation is that the Treasury Department put into the Social Security trust funds, out of general or income tax revenues, the different between what has been collected since 1974 in Social Security taxes and what would have been collected if unemployment had not exceeded 6 per cent.

This is the same idea used in so-called countercyclical revenue sharing to cities and states. The federal income tax is only used as a money source of last resort, when unemployment is high and revenues from other sources are corresponding low.

Not all this income tax money would be put into the trust funds in the first year; the payments would be phased, sources said, to reduce the drain on the treasury.

As it stood last night, Carter's plan would remove the wage ceiling from the taxes paid by employers in three steps over three years.

There has been considerable discussion within the administration over this proposal, but "as of today, it's in," one source said, adding that it could be dropped over the weekend. Other sources said its abandonment was unlikely.

Carter also is expected to propose, as part of his package, an increase from 7 to 7.5 per cent in the income tax of self-employed people.