President Reagan, reaching into the ranks of mainstream conservative economists, yesterday named Harvard professor Martin Feldstein to chair the President's Council of Economic Advisers.

The appointment signals a desire to bring into the White House a respected voice on economic policy not overly tinted by ideology, administration officials said.

At a time when many supply-side aspects of the Reagan economic program are under fire, White House aides said they felt it increasingly important to bring in an economist with a broad background who would not be a prisoner of any single theory.

"The important thing is that Feldstein is not exclusively theoretical," said one well-placed administration official who, along with others making economic policy for Reagan, says he believes the president was ill-served by the feuding between the theorists he appointed to key positions early in his term.

"He is tuned in to supply-side economics, but he is not a supply-side kook," said Herbert Stein, who served as chairman of the council in the Nixon-Ford years and now is affiliated with the American Enterprise Institute, a conservative think-tank here.

"He's been friendly to the generalized version of supply-side, that one expects tax cuts to work slowly," said James Duesenberry, also a Harvard economics professor and a council member in the Johnson administration. "He is not identified with the immediate response theory."

That is a reference to the claims made by supply-side economist Arthur Laffer, and others, that large income tax cuts would have a quick response in stimulating the economy and business investment. Critics of the theory point out that it has failed to bring about the promised short-term result.

Feldstein has more than once used the term "wizardry" to describe the contention by Laffer, a University of Southern California economist, that tax cuts will produce more revenues for the Treasury than they will cost.

Stein suggested that Feldstein might find the political environment in the Reagan administation "more receptive to him than in January, 1981, now that they recognize the more eccentric and ideological theories have not panned out."

Feldstein "is a person who will recognize what the function of the job is, and the function is not to sell your own ideas," said Stein. "He is not a missionary."

Benjamin M. Friedman, also a Harvard economics professor, said yesterday that the Feldstein appointment "brings into the administration a vastly higher level of capability than they have had so far. He is a vastly more talented and capable person than any economist they've had."

Feldstein, 42, president of the National Bureau of Economic Research, has written extensively on taxes, the Social Security system and unemployment--all topics with which the White House expects to be grappling in the next year.

"The president is satisfied he is the best man for the job," said deputy White House press secretary Larry Speakes. Reagan approved the appointment Thursday and interviewed Feldstein shortly afterward, Speakes said.

If confirmed by the Senate, Feldstein will replace Murray Weidenbaum, who resigned last month to return to Washington University in St. Louis. The mild-mannered Weidenbaum often seemed caught in the ideological cross-fire that dominated the early months of the Reagan presidency.

Feldstein has differed with Reagan by arguing for a delay in the 10 percent cut in income tax rates due next year, as a way to narrow the federal budget deficit. But he has pushed generally in the same tax-cutting direction that Reagan did during his first year. Feldstein has also argued recently that Social Security benefits can be trimmed for middle- and upper-income recipients.

Feldstein plans to take a two-year leave of absence from Harvard. He will head a three-member council dedicated largely to analyzing economic data for the president and making forecasts.

A second vacancy on the council will be filled by William Poole, a monetary policy expert at Brown University, replacing Jerry Jordan, who resigned. The third council member is William Niskanen, a former Ford Motor Co. economist.