U.S. District Court Judge Thomas A. Flannery, a former prosecutor who has served as a federal judge here since late 1971, has decided to take semiretired senior status when he turns 67 on May 10.

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. Five of the judges were appointed by President Carter.

Flannery, who was appointed by President Nixon, said he decided to take senior status as soon as he became eligible for it "so I can slow down somewhat . . . and have more control over my life. I won't have to just sit here and take one case after another as they come up."

He said he also expects to become more active in the U.S. Judicial Conference, composed of federal judges, for which he is chairman of two committees and sits on a third. Recently, Chief Justice Warren Burger sent Flannery's name to the White House as a possible member of a new congressionally mandated commission on sentencing.

The son of a carpenter, Flannery was born in Washington and grew up near North Capitol Street in a working-class neighborhood then known as Swampoodle. He graduated from Gonzaga High School, just a block from his home, and studied law at night without ever going to college. Flannery graduated in 1940 from Columbus University Law School, now part of Catholic University.

"I'm from the old Washington when it was like a small, sleepy Southern town," Flannery said in an interview yesterday. "It's much different now."

A tall, soft-spoken man, Flannery tried more than 300 cases before juries in his 12 years as an assistant U.S. attorney here. After seven years with the Washington law firm of Hamilton and Hamilton, he was appointed to the U.S. attorney's post by Nixon in 1969 and named a federal judge two years later.

Just after he was nominated in 1971 he described his views on the law as "conservative by modern day standards." But on the bench he has been widely regarded as a moderate and yesterday was praised as fair-minded by both prosecutors and defense attorneys.

Although he is Catholic and serves as a lector at the Little Flower Church in Bethesda, Flannery issued an injunction in 1983 against regulations that would have required federally funded agencies to notify parents when their children received contraceptives.

His most prominent case was in North Carolina where he served in 1983 and 1984 on special assignment as judge in the trial of nine Nazis and Ku Klux Klan members charged with civil rights violations that stemmed from a 1979 anti-Klan rally in Greensboro. Five Communists were killed and others wounded. The jury acquitted the defendants of all charges.

As a senior judge, Flannery can choose how many cases he wishes to handle and what types. He will continue to receive a judge's full salary of $76,000 a year.

Flannery and his wife, Rita, live in Bethesda and have two grown children, one a probation officer in D.C. Superior Court, the other a teacher at a Catholic school.