Justice Department investigators have found no evidence to support allegations that U.N. Ambassador Andrew Young improperly used his influence to help a federal prisoner, a department spokesman said yesterday.

"The allegations were found to be without substantiation," said spokesman Robert Havel.

Attorney General Griffin B. Bell has been told by the U.S. attorney in Atlanta, William Harper, that Young "is not a target of any investigation by the Justice Department," Havel added.

The allegaions involve Michael Thevis, and Atlanta businessman convicted of transporting obscene material across state lines and conspiracy to commit arson.

Several months sgo, columnist Jack Anderson reported allegations that Thevis paid Young money in exchange for Young's help in arranging Thevis' transfer to a federal prison hospital.

Young had denied any wrongdoing. He has said that while he was a congressman from Georgia, he had asked federal authorities to arrange a medical furlough for Thevis.

But he said the action was part of the normal service a member of the House provides constituents and that he took te action for humanitarian reasons.

The New York Daily News reported yesterday that one of Tevis' fellow inmates has told authorities that Thevis told him he funneled $250,000 into President Carter's 1976 campaign through Young.

The Justice Department spokesman said that although the allegations against Young were found to be unsubstantiated, other aspects, including the claimed campaign contribution remain "under active investigation" by the FBI, the U.S. attorney's office in Atlanta and department officials.