A major new provision in President Reagan's tax plan would recapture $57 billion worth of "windfall" gains companies would get from the lower corporate tax rates in the president's proposal, according to administration sources.

While affecting only about one U.S. company in 10, the provision generally would hit hardest at large, profitable firms that made major capital investments during the last five years.

The provision, not included in the original Treasury tax proposal (dubbed Treasury I), would effectively maintain the current corporate rate of 46 percent for all the income that firms deferred during that period through accelerated depreciation.

Without such a recapture provision, companies would pay tax on the deferred income at the new lower rate of 33 percent instead of the 46 percent rate that has been in effect since 1978.

Here is how the recapture provision would work:

Companies would add up the depreciation claimed for tax purposes on investments in capital assets made between Jan. 1, 1980, and Dec. 31, 1985. Companies with less than $400,000 in total depreciation during that period would be exempt from the recapture provision.

If the amount were more than $400,000, the corporations would add up the depreciation allowances they used in calculating their business expenses shown in their annual reports and statements to the Securities and Exchange Commission. Because those figures usually are based on so-called straight-line rather than accelerated depreciation schedules -- that is, there is no bunching of the write-offs in the early years that the assets are in use -- this figure usually is smaller than the depreciation number reported to the IRS.

If the difference between the excess depreciation claimed for tax purposes over that used in the financial reports totaled less than $300,000, corporations also would be exempt from the recapture provision.

The firms still subject to the recapture would include as income in 1986 and 1987 12 percent of the difference between the two depreciation figures less the $300,000. The percentage would rise to 16 percent in 1988. That addition to income -- 40 percent of excess depreciation altogether -- would be taxed at the new corporate rate of 33 percent. The effect would be the same as if all the excess depreciation were taxed at 13 percent, the difference between the old and new corporate rates.

The proposed change would raise substantial amounts of federal revenue: $7.6 billion in 1986, $19.4 billion in 1987, $20.4 billion in 1988 and $9.1 billion in 1989.

It would have roughly the same effect as reducing the corporate rate in stages rather than right away, the sources said. The option of reducing the rate gradually was presented to Reagan, but he rejected it in favor of the windfall recapture provision, they said.

The provision was included because of an overwhelming need to raise more revenue, sources said.

The president's plan contains a new depreciation system, called Capital Cost Recovery System (and nicknamed "crackers"), which would have six classes of assets rather than the four in current law or the seven proposed in Treasury I.

The original cost basis of assets would be indexed for inflation as was proposed in Treasury I. Depreciation currently is based on historical costs, which are not indexed.

Most assets that now can be depreciated over three years, such as automobiles, could be written off at a rate of 55 percent per year, and "closed out" -- written off entirely -- at the end of four years. (In Treasury I, the rate was 32 percent and the close-out period five years.)

Computers, trucks and buses, now being written off in five years, could be written off at a 44 percent rate and closed out in five years. (Treasury I, 24 percent and eight years.)

Aircraft, tractors, construction machinery and mining and oilfield machinery -- also now depreciable over five years -- could be written off at 33 percent per year and closed out in six years. (Treasury I: 18 percent and 12 years.)

The rate for other equipment now in the five-year group would be 22 percent and the period seven years. (Treasury I: 12 percent and 17 years.) Railroads and ships, now in the same group, would be written off at 17 percent and closed out at 10 years. (Treasury I: 8 percent and 25 years.)

Structures such as apartment buildings, which now are written off over an 18-year period, would be depreciated at 4 percent and closed out in 28 years. (Treasury I: 3 percent and 63 years.)

A separate revenue-raising change would remove an exemption proposed in Treasury I under which companies that entered into binding contracts to buy equipment or machinery could take accelerated depreciation and the investment tax credit even if the purchase arrived after the new plan's effective date. Congress is considered likely, however, to make any transition easier, leaving a revenue gap that must be filled.