The Watergate Special Prosecution Force is going out of business this week and Charles Ruff, the fourth and final Watergate prosecutor, says that he is tired, drained and frankly sick of it all.

After 20 months as the part-time, caretaker prosecutor, Ruff said, "I'm going to try and get these damn boxes packed as fast as I can and get the hell out of here . . . I am, for the record, sick of it. I look forward to leaving this office."

If Watergate was a sickness, then investigating Watergate - at least for too long - is itself a kind of sickness, according to Ruff.

Ruff appears strung out. "You work so damned hard at detaching yourself from emotional reactions," he said, "you can't do anything but come away almost artificially detached from the real world."

And if anyone will believe it, he says that there are no big secrets left to be uncovered. Should all the files and tapes be made public, people would be "titillated but not stunned," he said in a 1 1/2-hour interview two weeks ago which was embargoed for use today just before his office shuts its doors permanently.

Ruff says that he is pretty sure there is nothing monumental left and continuing to look under rocks and through former President Nixon's tapes will have no meaning to society or government.

Sure, Ruff said, there are a bundle of unanswered questions - but not any that would make a difference to history.

He said, however, he thought large parts of the tapes could be made public without violating the privacy of Nixon or others.

"I think everybody walked out of this office, some more, some less, with that incredible feeling of frustration that there were some things out there that if we only had the right person with the right document then we would know something else," he said.

The biggest such frustration, he said, was the long investigation into the $100,000 cash gift billionaire Howard Hughes made to Nixon friend Charles G. (Bebe) Rebozo. It resulted in no prosecution and lots of lawyers, Ruff said, "left here shaking their heads and with really deep concerns" over the case.

Nonetheless he said emphatically: "We ought to get on to other things." Ruff, 37, is taking a job in the Carter administration as deputy inspector general in the Department of Health, Education and Welfare.

Ruff has held his post on a part-time basis while also maintaining his position as an associate professor at Georgetown Law Center. He is the least known of the four men who have held the post of Watergate special prosecutor. The others were Archibald Cox, Leon Jaworski and Henry Ruth, whom Ruff succeeded in October, 1975.

Ruff has been confined to a wheelchair since an illness crippled his legs in 1964. In the 14 years since he graduated from Law School (Columbia, 12th in a class of 235), his career has intermittently been that of law professor and Justice Department prosecutor.

He is breaking some of the most sacred Washington bureaucratic rules - he is glad to eliminate his job and tell people it is not needed. He also is returning a portion of his office's $2 million budget for fiscal year 1976.

Ruff is also breaking some Watergate traditions. He promises not to write a book and the 74-page report he issued with his departure adds no new revelations or even interesting tidbits to the reocrd.

Because the Watergate scandal had such dramatic impact and because of the lingering questions in some people's minds. Ruff said that he has a nightmare that in 15 or 20 years there might be another congressional investigation of Watergate.

Depending on what happens in the next two decades and what political party is in power, Ruff said it may all look different to future congressional investigators:

"If they ever get into our files they will go in and perhaps say, 'God, there's this piece of paper and they didn't do anything with it,' or that This reveals to us the ultimate secret of what was going on.'"

Ruff continued) "This office came into being with people writing stories about what a marvelous bunch of lawyers." The press has perhaps, he said, given the office "unduly favorable reviews," and in the years to come there may likely develop another view.

"I suspect that we'll come off better than the Warren Commission that investigated the Kennedy assassination," Ruff said, "and I suspect we'll come off better than the FBI and CIA in the (Martin Luther) King assination case, but there are judbment calls that were made that people can legitimately question."

Ruff said that there is nothing he has done to protect himself from a more hostile view of the work of his office.

If called to testify some day at such an inquiry, Ruff said he knows just what to do. "I'd say, 'Gee, I just don't remember what happened back then, and they won't be able to indict me for perjury and that, maybe, that's the principal thing that I've learned in four years . . . I just intend to rely on that failure of memory."

Ruff said that he is adamantly opposed to a permanent special prosecutor, maintaining that the potential abuse by such an office is greater than its need.

In fact, Ruff went so far as to say that the Justice Department should in the future be able to investigate impartially even the President's closest aides and friends.

A special prosecutor would only be needed in the extreme case where the President or the Attorney General were the subjects of a criminal investigation, acording to Ruff.

This conclusion, he said is based not on any particular confidence in the inlegrity of the new Carter administration but his conviction that the Nixon administration and Watergate were aberrations and will not recur in his lifetime.

Ruff made these additional points:

The existence of the special prosecutor's office has hurt "morale, the public image and the objective competency" of the Justic Department, which "for the last four years has had this sort of strange appendage tacked on to it that's done all the really hot and sexy work." Accordingly, the department can return to its position as the pre-eminent law enforcement agency only when the special prosecutor's office is closed.

is office "barely scratched the surface on" the alleged "sale" of ambassadorships to political campaign donors. "Aren't our foreign relations amore important than that?"

He does not view as "a great achieverational version in the testimony" on the original motivations for the Watergate burglary: to obtain political intelligence on the Democrats. "Even if that's that's not the real reason . . . I don't really care what the ultimate answer is, because I don't think it's going to make any difference."

He does not view the downfall of Richard M. Nixon as "a great achievement (for this office) . . . I view it as a tragedy. But the system worked and that is encouraging, not the fact that Richard Nixon is no longer President."

On the prison sentences some of the Watergate figures received: "If a prosecutor sees the incarceration of a human being as a great achievement then he's just not doing his job. He's got to say, Was it done according to the rules?' and 'Was some semblance of justice done?'"

Leon Jaworski, the second Watergate special prosecutor, should not have used certain confidential material in his best-seller book, "The Right and the Poser." "I think there were things reprinted in Leon's book that were not otherwise available in a public forum, and would not have been disclosed by this office under any other circumstances, and I think they should not have been in his book . . ."

He had a high opinion of two other books written by former Watergate prosecution staffers. Ruff said the Richard Benveniste and George Frampton's book, "Stonewall," "provides the best lawyer's insight into what was going on," and James Doyle's book, "Not Above the Law," "provides the most human insight into what was going on at this time."

He felt under pressure during last year's investigation into allegations the President Ford had improperly diverted maritime union campaign contributions for his personal use. Ruff issued a statement clearing Ford before the presidential election. He said his investigation might have gone on a few more weeks if it had not surfaced during the election campaign. There was, he said, "that incredible emotional tug which says Jesus Christ let me get out of here before we're on election day!" On the other hadn, Ruff said, "One thing I sure didn't want to have happen was to wake up the morning after I closed the investigation to have somebody writing a story saying, hey, I just discovered over here this magnificent piece of information."

The request to investigate the Ford allegation, Ruff said, "represents one prime reason why we should not have a special prosecutor floating around on a permanent basis . . . we were a tempting repository for that kind of problem." And, he said, the Justice Department is going to have a learn to deal with such "ultimate tough problems" on its own.

Nixon could have received a fair trial if he had been indicted and not pardoned, Ruff said. Nixon could have avoided resigning, he added, "if he could have brough himself to the point of public embarrassment involved in a straightforward expanation even as late as 1974, if he had given a sort of mea culpa kind of speech, even then he could have saved himself."