The U.S. Court of Appeals yesterday sharply criticized the Library of Congress for subjecting a low-level employe to an FBI investigation of his background and loyalty because he attended meetings of a legal, socialist political group.

The court ordered the case, which was decided against the employe by District Judge Thomas P. Jackson, sent back for a new trial.

The worker, Harry Kenneth Clark, was a part-time reshelver of books at the library and a full-time college student when, in 1975, several informers told the FBI that Clark was attending meetings of the Young Socialist Alliance, an affiliate of the Socialist Workers Party. Both groups are legal.

At the library's request, the FBI later conducted a full field investigation of Clark, including his religious affiliations and whether Clark was a homosexual, the court said.

The investigation, which caused Clark "acute embarrassment," according to the appellate decision, failed to show that Clark was a security risk. But when Clark graduated from college and applied for 40 full-time positions at the library, the court said, he was turned down.

"The library failed completely . . . to demonstrate that Clark posed any national security risk," the appeals panel said.

The panel said the library was unable to show "the existence of any legitimate, much less compelling justification" for the investigation.

". . . to hold that the full field investigation of Clark was justified . . . would be to sanction intrusive and injurious investigations initiated on the basis of minimal information," the court said.

The decision was written by District Judge Edmund L. Palmieri of New York and joined in by Chief Circuit Judge Spottswood W. Robinson III and Circuit Judge Patricia M. Wald.