President Reagan formally announced yesterday his intention to name Murray L. Weidenbaum, a professor at Washington University and an expert on business deregulation, as chairman of the Council of Economic Advisers.

In another action, the president nominated R. T. "Tim" McNamar, executive vice president of Beneficial Standard Corp. who has played a personnel role in the transition, as deputy Treasury secretary.

Weidenbaum, 53, whose expected nomination was reported earlier, served as one of Reagan's economic advisers during the presidential campaign and headed a transition team on regulatory issues.

In a briefing for reporters at the White House yesterday, Weidenbaum sought to squelch reports that the CEA will be downgraded in the new administration -- reports which circulated when appointments to the CEA were not forthcoming. "I took this job because President Reagan asked me to be the chief economic adviser to the president," he said.

McNamar, 41, named to the No. 2 job at Treasury, is no stranger to Washington. A lawyer with a Master's degree in business administration from Dartmouth's Amos Tuck School of Business Administration, McNamar served as executive director of the Federal Trade Commission from November 1973 until March 1977.

From January 1972 until joining the FTC, he served first as director of the Office of Economic Policy and case analyst for the Pay Board and then as an internal management consultant to John T. Dunlop, director of the Cost of Living Council. Before coming to Washington, he was a management consultant for 5 1/2 years with McKinsey & Co. Inc. in San Francisco.

At the FTC, McNamar was responsible for executive and administrative supervision over all offices, bureaus and staff. In that capacity, he developed a computer system for tracking the cases and programs of the commission and introduced a program-budgeting concept at the agency.

"I think he's a superb choice," Lewis A. Engman, former FTC chairman and McNamar's boss at the FTC, said yesterday. "He's an excellent administrator . . . and he has the kind of knowledge of Washington that will mesh well with the new secretary [Donald T. Regan]."

Since leaving Washington, he has been executive vice president and chief financial officer of Beneficial Standard Corp., a diversified financial services holding company based in Los Angeles. The company, with assets of about $550 million, has subsidiaries whose activities include real estate development and life, property and casualty insurance.

McNamar will be the youngest deputy secretary in the history of the Treasury Department, a Treasury spokesman said yesterday.