The New York Post said Bloomberg made the remarks after it reported the city's police department was preparing for potential violence at the Aug. 30 event in Jamaica, Queens.
The rapper and former crack dealer, whose real name is Curtis Jackson, has a habit of wearing a bulletproof vest in public following a 2002 incident in the neighborhood in which he was shot nine times.
Law-enforcement sources told the newspaper they feared someone might try to assault him again if he performed in public there, but Bloomberg said the rapper isn't scheduled to take the stage.
"Our understanding after talking with 50 Cent is he has no plans to perform whatsoever. He might go, but he's not going to perform," the Post quoted Bloomberg as saying.