U.S. District Court Judge Barrington D. Parker, who has presided over some of Washington's most well-known cases including the trial of presidential assailant John W. Hinckley Jr., has decided to take semiretired senior status when he completes 16 years on the bench on Dec. 19.

The decision opens the way for President Reagan to fill another vacancy on the 15-member court, which now has three Reagan appointees and one nomination pending, that of Stanley Sporkin, the Central Intelligence Agency's general counsel. Yesterday Reagan nominated George Revercomb, 56, of McLean, now a D.C. Superior Court judge, to succeed Thomas Flannery, on the District Court bench. Five of the judges were appointed by President Carter.

Parker, who will turn 70 on Nov. 17, was appointed by President Nixon.

"I thought 16 years was a pretty good record of having devoted my activities to the bench," Parker said yesterday. He added that he intends "to remain active as a senior judge" although he will be able to cut back on his workload and choose the type of cases that he tries.

A lifelong Republican, Parker has gained a reputation as a sometimes crusty, highly independent judge whose rulings have sometimes nettled prosecutors and defense attorneys alike.

During the trial of Hinckley, who shot and wounded President Reagan, his press secretary James Brady, a policeman and a U.S. Secret Service agent, Parker charged the jury that the defendant could be convicted only if the government proved his ability to obey the law and appreciate the wrongfulness of his acts beyond a reasonable doubt.

Hinckley was found not guilty by reason of insanity, and Parker committed him indefinitely to St. Elizabeths Hospital, where he remains.

In a television interview later, Parker said he had received more than 1,500 letters, most blaming him for the acquittal. Parker, who is black, said some of the letters had "a nasty streak of racism running through them."

Among the other celebrated cases he tried were those of Richard Helms, the former CIA director accused of lying to a Senate committee; former representative Otto E. Passman (D-La.), charged with accepting a bribe from a South Korean businessman; and the two Cuban exiles and Chilean security agent accused of murdering Orlando Letelier, a former Chilean ambassador.

Born in Rosslyn, Parker is the son of a lawyer who was a part-time preacher. His wife, Marjorie H. Parker, is a former member of the D.C. City Council and former chairman of the board of the University of the District of Columbia.