President Reagan has completed the selection of key economic advisers by choosing Murray L. Weidenbaum, a professor at Washington University and an expert on business deregulation, to head the president's Council of Economic Advisers, according to a Reagan White House official.

Weidenbaum, 53, one of Reagan's economic advisers in the presidential campaign, headed a transition team on regulatory issues and was regarded as a top contender for a senior post in the new administration. The appointment of the chairman and other two members of the CEA dragged on throughout the transition period, however, for reasons that still are unclear. Some saw the delay as reflecting a possible downgrading of the CEA chairman's post in the Reagan administration.

Weidenbaum, however, is said to be satisfied about his CEA role following a meeting Monday with Reagan on the structure of administration economic policymaking and specific policy issues. As chairman of the CEA, Weidenbaum will be chief economic adviser to the president and the Cabinet, sources said. The new Treasury secretary, Donald T. Regan, former chairman of Merrill Lynch Inc., has been designated chief economic spokesman for the administration. The remaining two positions on the CEA have not yet been filled.

An economist with the old Bureau of the Budget from 1949-1957, Weidenbaum served in the Nixon administration as assistant secretary of the Treasury for economic policy from 1969-1971 under then-Treasury secretary John Connolly. (Weidenbaum supported Connolly's unsuccessful campaign for the Republican presidential nomination last year. In 1971 Weidenbaum moved to Washington University to establish the Center for the Study of American Business, which he now directs. It was in that position that he gained a reputation as a critic of federal regulation. Among economists, Weidenbaum is regarded as pragmatic and witty, with an ability to work well with those of differing views.

Weidenbaum often has advocated a one-year moratorium on new federal regulations, giving policy makers time to pinpoint changes in existing rules, but it isn't known whether he has urged Reagan to follow that approach now.

Weidenbaum also has said he expects Reagan to issue an executive order requiring federal agencies to weigh the costs and benefits of proposed major regulations before issuing them, except where cost-benefit analysis is specifically forbidden by law.