The Reagan administration expects its economic plan to boost business investment to a degree unparallelled in the nation's history, according to figures given Congress yesterday by Treasury Secretary Donald T. Regan.

Regan laid out a scenario for the Joint Economic Committee in which virtually all of the financial resources freed as federal spending and tax cuts end up financing business investment.

The Treasury secretary asserted that business investment will rise 11 percent a year faster than inflation for at least the next five years. Such an unprecedented increase would lift spending on new plants and equipment to more than 12 percent of gross national product by 1983 -- a lever never achieved by the U.S. economy -- from about 10.9 percent this year. In 1986, a huge 14.4 percent of GNP would be invested by business.

By far the most rapid growth in modern times occurred between 1961 and 1966 when business investment rose slightly more than 10 percent a year in real terms. However, investment in 1961 amounted to only 8.8 percent of GNP.

While the changes in percentages seem small, each 1 percent of GNP this year is equal to more than $29 billion. All of the sweeping budget cuts proposed by President Reagan, including many not yet indentified, will by 1986 reduce outlays by an amount equal to 4 percent of GNP.

The reaction of private economists to the investment figures ranged from astounded to surprised. In a typical reaction, one declared, "That's totally off the wall." Said another, "They have gone a hell of a lot further than I would."

Otto Eckstein of Data Resources Inc., for instance, said such a shift to investment "would require a massive restructuring of the American economy.It would have to be characterized as far beyond historical experience," he said.

Moreover, Eckstein declared, "You would run out of capital goods [production] capacity quickly . . . and construction wages would be going up 15 percent or 20 percent a year like they did in the early 1970s."

Regan, defending the program the president announced Wednesday, said the investment result is part of "an economic scenario . . . not a forecast in the conventional sense, projected by a traditional econometric model assuming no changes in people's behavior." Instead, the administration is counting on major changes in people's expectations and behavior.

Among the results seen in the scenario, Regan said, is a drop in consumer price inflation from 11 percent this year to just over 4 percent in 1986 and a rise in GNP, after adjustment for inflation, of between 4 percent and 5 percent over the same period. Thus, business investment is supposed to grow more than twice as fast as the overall economy.

Since virtually all business investment involves purchases of land, buildings and equipment, the implications of such a rapid growth in investment are immense.

For instance, while the administration intends to cut the federal budget by an amount equal to 4 percent of GNP by 1968, it will not actually free that amount of real resources for investment.

Only slightly more than one-third of the federal budget is spent buying goods and services. The rest of it goes for transfer payments, that is, Social Security benefits, pensions and the like. And since military spending, which will be rising not falling, accounts for the bulk government purchases of goods and services, it is likely that the federal demand on the nation's real -- as opposed to financial -- resources will decline by no more than 1 percent of GNP by 1986 under the Reagan plan. The drop could be even less, analysts said.

Investment on the scale envisioned by the administration, therefore, must be accomplished by reducing some other demands on the nation's resources, presumably from personal consumption.But if consumption is not rising strongly, Rep. Frederick Richmond (D-N.Y.) asked Regan, why will businessmen be in such a rush to invest?

"When you create more production, you create more demand" and therefore incentives to invest, Regan said.

Regan also stressed that the business and personal tax cuts in the administration program would quickly increase after-tax rates of return on investments, which he said would greatly, encourage more spending on new plants and equipment.