Judge Carl McGowan, long considered a moderating influence among those with sharply differing philosophies on the U.S. Court of Appeals here, has told the White House he plans to retire from fulltime service on Aug. 31 after 18 years as an appellate judge.

McGowan's decision, which came as a surprise to his colleagues on the bench, gives President Reagan his first opportunity to appoint a judge to the powerful appeals court here, which often is described as second only to the Supreme Court in its influence.

Court officials said yesterday that McGowan gave no reason for his decision to accept the semiretired status of "senior judge." McGowan was out of the country and could not be reached for comment. He became eligible for senior status last May, when he turned 70.

Former President Jimmy Carter made four appointments to the appellate court here, two of whom are women and all of whom were considered to have strong liberal credentials. In May, Reagan abolished Carter-sponsored judicial nominating commissions, which proposed candidates for the federal appeals courts throughout the country, contending that the commissions were inefficient and that political interests interfered with the merit selection process.

Instead, the administration has decided to conduct its own search for candidates within the judicial circuits around the country and consider suggestions made by members of Congress and the organized bar.

Lawyers who know McGowan, who was named to the court by President John F. Kennedy, have described him as a peacemaker, a gentle man with a persuasive intellect among often-warring liberal and conservative factions on the appeals court. His written opinions demonstrated his attention to clarity and common sense.

Last year, for example, he wrote the court's opinion resolving a long-standing complex dispute about how private lawyers should be paid when they win discrimination cases against the government.

Earlier in his term on the bench, writing in two criminal cases, McGowan set out limitations for use of a defendant's prior convictions at trial. McGowan also was far less predictable than is colleagues, lawyers said. For example, in a decision that surprised some observers, McGowan agreed with the court's freshman member, Harry T. Edwards, and reversed a defendant's murder conviction after 17 years. The full appeals court, in a split decision, declined to review the case, supporting Edwards and McGowan, and the case is nowin the U.S. Supreme Court.

Chief Judge Spottswood W. Robinson III, who took over the court after McGowan's brief tenure as chief judge earlier this year, said he and the other appeals court judges received a note from McGowan last Thursday announcing his plans. The chief judge of the appellate court here is the most senior judge on the court who is not over 70; judges can continue to serve after turning 70 but not as chief judge.

"Judge McGowan will go down in history as one of the great judges of this court," Robinson said yesterday. "He labored in the ablest possible fashion for more than 18 years and contributed much to the strength of the court."

"His participation as an active judge will be sorely missed but we take comfort in the fact that he will be continuing to serve as a senior judge," Robinson said.

A senior judge carries a reduced case load, continuing to participate in the court's first tier of decision-making by panels of three judges. However, a senior judge does not participate when cases are heard at the second level by the full 11-member appeals court.

An appointment to federal appeals court here is likely to be viewed as of particular importance by the Reagan administration, since the bulk of the court's caseload involves review of complex U.S. regulations passed by government agencies and often vigorously contested by private industry. Those nominated to the court often have been selected from throughout the country because of the broad nature of the court's caseload.

One possible replacement for McGowan mentioned in legal circles in University of Chicago Law School professor Antonin Scalia, a former high-ranking Justice Department official who was active on several Reagan transition teams.

"I can't say that I've talked to anybody with the authority to offer the job," Scalia, 45, said during a telephone interview. "I don't know if I'm under active consideration."

Scalia's area of expertise is administrative law, which deals extensively with interpretation of federal regulations, and constitutional law. He has been a resident scholar at the American Enterprise Institute, a conservative think-tank that has worked closely with Republicans, and was a member of a committee during the Reagan compaign that drew up plans for federal regulatory reform.

A Justice Department official said yesterday that the search has not yet started for McGowan's successor.