The federal tax rate on the domestic earnings of the nation's largest corporations fell sharply last year to 20.5 percent, and 33 companies with U.S. earnings in excess of $100 million paid no federal taxes whatsoever, according to a study by Tax Analysts, a nonprofit research organization.

Overall, the average effective rate on domestic income by corporations fell from 22.7 percent in 1980 to 20.5 percent, according to the study. This represents a 10 percent decline, at least part of which would be attributable to the Reagan administration's 1981 tax cut, which gave major tax breaks for new investments.

The conclusions of the study, which are likely to be disputed by corporate accountants and financial officers, come at a time when Congress is considering tax legislation -- strongly endorsed by President Reagan last night -- that would take back many of the business tax breaks enacted in 1981.

In the study of 514 of the nation's largest corporations, commercical banks, which benefit from a wide range of tax preferences, had a negative tax rate averaging 12.6 percent.

A negative rate does not mean that the companies got payments back from the federal government. Instead, it means that a company with a negative tax rate in 1981 will be able to reduce tax liabilities in future years.

In addition to commercial banks, Tax Analysts contended that transportation, shipping and railroads had negative tax rates on domestic income in 1981.

The highest tax rates were paid by publishing companies, 36.3 percent; cosmetic and soap firms, 39.4 percent; and by apparel manufacturers, 39.7 percent.

The study found that one of the most controversial parts of the 1981 tax cuts -- corporate tax sales through paper "leases" -- was not a significant factor in the decline in corporate tax rates. "Many firms did not utilize leasing in 1981, and leasing was not a major tax reduction factor even for those firms that did enter into leasing transaction."

In at least one case, however, the study, which was based on documents filed with the Securities and Exchange Commission, did not reveal the extent to which corporate tax sales reduced federal tax liabilities. The study found that General Electric, which used tax leasing to get what amounted to a $100 million refund in 1981, paid federal taxes at a rate of 25.1 percent on domestic earnings of $1.9 billion.

This failure to pick up the effects of tax leasing resulted from the fact that GE benefited from the leasing transactions of a subsidiary, General Electric Credit Corp., which was not clearly spelled out in disclosure statements.

In theory, the corporate tax rate is 46 percent, although companies use a host of credits, deductions and other items of tax preference to reduce their rates.

Tax Analysts uses a system of accounting to calculate tax rates that is disputed by many corporations. The most controversial accounting practice used in the study is to count deferred taxes as taxes not paid. Corporations generally contend that while the deferred taxes are not owed in the year reported, they will become payable in the future, and consequently count them in their own calculations of effective tax rates. For example, Tax Analysts contends that Northrop Corp., a major aerospace contractor, had a negative tax rate of 103.1 percent in 1981 despite U.S. earnings of $67.7 million. This calculation is primarily based on the assumption that the company was able to use a special tax break for long-term contractors to postpone payment of $89.6 million in federal taxes.

A spokesman for the firm contended, however, that Northrop had a positive tax rate of 29 percent, based on the assumption that deferred taxes, while not paid in the year incurred, will be liabilities in future years.

Similarly, Tax Analysts contends that Dow Chemical had a negative federal tax rate of 7.8 percent in 1981 on U.S. earnings of $516 million. A spokesman for Dow claimed the firm had a "positive rate" but declined to give a specific number.

Many of the lowest effective tax rates were found in capital intensive industries, which were the major beneficiaries of the 1981 tax bill. The rate for mining and crude oil production was found to be 9.4 percent; metal manufacturing, 10.1 percent; aerospace, 13.5 percent; chemicals, 13.6 percent, and automobiles, 19.1 percent.