The Reagan administration voiced its continuing opposition yesterday to the law providing for special prosecutors in cases involving high government officials, but conceded that a series of proposed changes would make it more palatable.

Associate Attorney General Rudolph W. Giuliani told an unsympathetic Senate subcommittee that the Justice Department would rather have legislation leaving it entirely up to the attorney general to decide whether an independent investigation is needed in sensitive cases.

But Giuliani acknowledged that the changes proposed by a bipartisan subcommittee majority--Chairman William S. Cohen (R-Maine), Warren B. Rudman (R-N. H.) and Carl Levin (D-Mich.)--would constitute "a very major improvement" in the four-year-old statute.

Cohen said hearings last year "revealed substantial weaknesses" in the law, part of the Ethics in Government Act of 1978, especially in the range of officials it covers and in the "very low standards for triggering a preliminary investigation and for appointing a special prosecutor" on completion of the preliminary inquiry.

But Cohen and his colleagues disagreed sharply with Giuliani's contention that the basic law ought to be repealed. Giuliani complained that it rests on the debilitating presumption that the Justice Department "cannot be trusted to investigate and prosecute certain cases and certain individuals."

"The issue really isn't, 'We don't trust you, we think you're a bunch of crooks,' " Sen. Thomas F. Eagleton (D-Mo.) told Giuliani. "The issue, really, is the appearance of impropriety. That's what this legislation is all about."

Under the present law, which expires in October of 1983, the attorney general must order a preliminary investigation whenever he receives "specific information" that a covered federal official or campaign official has commited a federal crime, other than a petty offense. After 90 days, the attorney general must either seek court appointment of a special prosecutor or certify that the matter is "so unsubstantiated that no further investigation or prosecution is warranted."

The proposed changes would call for a preliminary investigation only on receipt of what the attorney general deems "information sufficient to constitute grounds to investigate." After a maximum of 150 days, appointment of an "independent counsel" would be required only when the attorney general finds "reasonable grounds to believe that further investigation or prosecution is warranted."

The amendments would also extend the law for five years and change its coverage to include the president's immediate family and close in-laws, and to exclude middle-level White House executives below the $60,000-a-year mark.

Other witnesses included former White House counsel Lloyd N. Cutler and the American Bar Association's Robert D. Evans. Both praised most of the changes.

The subcommittee also heard from former attorney general Elliot L. Richardson, who appointed the first special prosecutor in modern times, Archibald Cox, on his own authority in 1973 in the midst of the Watergate scandal. President Nixon later forced Richardson to resign for refusing to fire Cox.

Richardson welcomed the proposed changes, but professed a basic distaste for the law which, he said, should be called "the 'No Ethics in Government Act' because it presumes there are none." He argued that the lesson to be drawn from Watergate is "not that so many people did wrong, but that so many people (such as Sen. Cohen, then a member of the House Judiciary Committee) did right."

Cohen said it was still "a rather near thing." He reminded Richardson that "it required your resignation at one point to keep things going."