FOLLOWING UP on the "serious reservations" expressed by Attorney General William French Smith a month ago about the constitutionality of the special prosecutor law, the Deaprtment of Justice has recommended a series of major changes in this post-Watergate legislation. Its recommendations, however, are secondary to it major request. The department would prefer that Congress repeal the law and strongly opposes extending it beyond its present expiration date in 1983.

This resistance to the continuation of the special prosecutor's office was to be expected. The implication of its existence is that there are certain kinds of cases in which the Department of Justice and the attorney general, whoever he may be, cannot be trusted. This has to be annoying to those who work there, because most of the time this questioning of departmental and personal integrity is unjustified. But on those rare occasions when the implication is justified, as was true during the Watergate years, the need for a special prosecutor is overwhelming.

This means that Congress should brush aside the suggestion that the law be repealed -- in fact, Congress should eliminate the expiration date -- and get on with the business of making many of the changes the department has recommended. Three years of experience with the law make clear that it covers both too many and too few people for too long a time and its provisions for the appointment of a special prosecutor are triggered too easily.

Associate Attorney General Rudolph Giuliani has recommended to a Senate subcommittee that the law be narrowed in some respects and broadened in others. He wants to eliminate it automatic applications to scores of named ranking government officials but expand its application to any official or private citizen whose personal, business or political ties to high officials seem to present a conflict of interest if the Department of Justice investigates them. This makes sense, as does his proposal to limit the law's application to felonies (it now covers all federal crimes) and to permit the attorney general to wash out allegations that would not be investigated if they were made against ordinary citizens.

Mr. Giuliani's other major proposal is that the power to appoint and remove special prosecutors be vested in the attorney general instead of in a special three-judge federal court as it now is. He makes a powerful argument that the present arrangement is unconstitutional as a breach of separation of powers. But the memory of the Saturday Night Massacre, when President Nixon insisted that a special Watergate prosecutor be fired, is still fresh enought to raise doubts about Mr. Giuliani's solution.

Perhaps the Senate subcommittee can find an arrangement somewhere between these two systems that would reduce the constitutional problem without curtailing so sharply the independence that a special prosecutor needs if he is to be fully effective. It would be worth the effort because the special prosecutor arrangment, despite the discomfort it has brought to the Department of Justice and, unnecessarily, to two of President Carter's close aides, has helped restore public confidence in the integrity of the federal government.