President Reagan yesterday nominated Treasury Undersecretary Beryl Sprinkel to be chairman of the Council of Economic Advisers, an organization the president once considered abolishing but since has decided to retain.

White House spokesman Larry Speakes said Reagan soon will fill the other two vacancies on the CEA and build up its staff of 20 economists and statisticians.

"The president wishes to get the CEA at full strength so he can rely on them through Cabinet councils and other special economic groups," Speakes said. He said that, because the council is a "statutory creation, we couldn't abolish it, so the president decided to use it to the fullest extent."

Sprinkel's nomination is subject to Senate confirmation.

Sprinkel, a monetarist, would replace Martin S. Feldstein, who resigned last July to return to teaching at Harvard University after a stormy tenure marked by disagreement with the president and most of his other top economic advisers, including then-Treasury secretary Donald T. Regan, over the negative effects of the federal budget deficits. Feldstein's predecessor, Murray L. Weidenbaum, also frequently was at odds with the administration.

The only member of the CEA now is William A. Niskanen Jr., who will return to UCLA. Reagan asked Niskanen to stay on, but he said he would do so only if he were named chairman, administration sources said. William Poole, the third member of the CEA, returned to Brown University on Feb. 1.

Speakes said that Treasury Secretary James A. Baker III will continue to be the administration's chief economic spokesman but that Sprinkel will report directly to the president, not to Baker.

"He will be economic adviser to the president. This is the top economic position attached to the White House," Speakes said.

The CEA will produce the president's annual economic report and economic assumptions for budgeting and will monitor economic developments and indicators, Speakes added.

Sprinkel, 61, is a native of Missouri who got his Ph.D. in economics at the University of Chicago, where he became a disciple of Milton Friedman, one of the nation's leading monetarists.

He taught economics at Chicago and was executive vice president and economic adviser at the Harris Trust and Savings Bank in Chicago before joining the administration as undersecretary of the Treasury for monetary affairs in March 1981. He was responsible for formulating and implementing U.S. international monetary policy, Treasury involvement with international lending institutions, managing the federal debt and coordination with the Federal Reserve Board.

In the latter role, he criticized the Fed for not keeping the growth of the money supply within its own target ranges. As a monetarist, he believes that the money supply strongly affects economic performance.

In his first two years, Sprinkel's outspokenness earned rebukes from Regan, but he is seen now as a loyal team player, and he campaigned for Reagan last year.

Speakes said Treasury officials indicated that "it is not likely that Sprinkel's post at Treasury will be filled as it is currently constituted. They want a different configuration. They want to redo their functions over there."

In an interview last December with the conservative weekly Human Events, Reagan raised the possibility of abolishing the CEA and relying on the Treasury Department for economic advice.

Despite Reagan's decision to build up the CEA, the fiscal 1986 budget request for the council is $2.3 million, about $200,000 less than for fiscal 1985. Administration officials said the difference is "negligible."

The council's staff includes 12 senior staff economists, six junior economists, a senior statistician and a special assistant to the chairman.