A man who tried to commit suicide by throwing himself in front of an oncoming subway train has won a $650,000 settlement from the New York City Transit Authority.

The settlement was offered by the Transit Authority and accepted by the lawyer for the man as a jury was being selected Tuesday for the case in State Supreme Court in Manhattan.

Richard Bernard, general counsel for the Transit Authority, characterized the settlement as ''fair'' for both the man, 26-year-old Milo Stephens, and for the Transit Authority. Mr. Stephens lost a leg, an arm and part of another arm in the accident. He and his lawyer, Aaron Broder, had intended to contend that the motorman had demonstrated some negligence in not stopping sooner.

Mr. Bernard explained that the decision to settle was made by the Transit Authority because of the doctrine of comparative negligence that exists in New York State law. Negligence by Operator