A meteoric rise of China to second place in world economic output in the next two decades and stagnation of the Soviet economy to a poor fourth are startling projections from the report of the Commission on Integrated Long-Term Strategy, which was made public over the weekend and is to be presented to President Reagan today.

Because "in some measure, military power reflects economic power," the blue-ribbon advisory commission said, such changes, should they occur as predicted, are likely to have major repercussions for the United States and the global strategic environment in the next century.

China "may well become a superpower, in military terms" according to the report. A "key question" is whether Japan will exercise its option to match its economic power with military power. And the Soviet Union's stagnation, as well as Soviet attempts to reverse it, are "huge imponderables for U.S. defense planners," the commission said.

The projected rise of China in the economic field is a particular surprise. As recently as 1980, according to Rand Corp. figures on which the commission report is based, China's economy in dollar terms was only about half the size of Japan's and a little more than one-third the size of the Soviet Union's.

One reason for the prominence of China is that the commission's working group on the "future security environment" converted Chinese currency to dollars using a "purchasing power parity" formula which triples the dollar value of the Chinese economy compared to the usual "exchange rate" method.

Charles Wolf, director of Rand's International Economic Policy research program and chairman of the commission working group, said the formula used is generally considered more accurate. One well-informed economist, who asked not to be quoted by name, said the World Bank and International Monetary Fund have considered using such a formula to depict China's economy in dollar terms, but that the Chinese government protested -- evidently because it would make the Middle Kingdom a wealthier country no longer eligible for "soft loan" benefits from development institutions.

The commission calculates, based on a variety of demographic, employment, investment and productivity data, that the Chinese economy is likely to grow at the relatively rapid rate of 4.6 percent yearly over the next 20 years.

Harry Harding of the Brookings Institution, author of a recent study of China's economic and political reforms, said commission's projections of Chinese output are higher than others he has seen, but that they might be achieved if one assumes a continuing rapid growth rate.

Most observers expect Chinese military spending to grow after being held back rigorously in the past several years. "China will have the potential to spend a great deal more {on the military}, but we don't know what their level of technology will be or what military decisions will be made," Harding said.

The U.S. annual growth rate was calculated by the commission working group to be about 2.5 percent over the next two decades. This is considered a minimal-growth figure by many economists, just keeping ahead of the growth in the labor force, but it would leave the United States still far ahead of all others in total output in 2010.

Japan's powerful economy surpassed that of the Soviet Union, in dollar terms in the last year, according to Wolf, partly due to the rise of the yen against the dollar. The working group calculated that Japan's future growth will decline slowly: 3.7 percent yearly until 1990, 3.0 percent from 1990 to 2000 and 2.7 percent from 2000 to 2010.

Although China is predicted to have a slightly larger GNP than Japan in the year 2010, China will also have well above 1 billion people to feed and clothe. This will soak up a large proportion of China's national output, while Japan's population will be many times smaller.

The relative decline of the Soviet Union to a poor fourth in national economic power is based on a Central Intelligence Agency estimate that the Soviet economy was about 60 percent of the size of the U.S. economy in 1976, and the working group's calculation that Soviet GNP will grow at only 1.6 percent yearly over the next two decades.

This calculation assumes that Soviet leader Mikhail Gorbachev's "perestroika" restructuring campaign to spur Soviet growth succeeds to some extent, but from a low current base. The working group adopted this estimate, while offering calculations that Gorbachev will do far better -- or far worse.

"Whatever the long-term prospects for Soviet economic growth, progress in the near term is apt to be modest," the report said.