The invention discloses a prefabricated assembly type bridge multi-span synchronous installing bridge girder erection machine and an installation method thereof. The multi-span synchronous installingbridge girder erection machine comprises a girder, a rear main overhead travelling crane, a middle main overhead travelling crane, a front main overhead travelling crane, a rear supporting leg, a first middle supporting leg, a second middle supporting leg, a first front supporting leg and a second front supporting leg, wherein the rear main overhead travelling crane, the middle main overhead travelling crane, and the front main overhead travelling crane can lift heavy type components; the rear supporting leg supports on a bridge floor; the first middle supporting leg and the second middle supporting leg are fixed to a supporting structure; and the first front supporting leg and the second front supporting leg supports on a pier body corbel or a bearing platform. According to the prefabricated assembly type bridge multi-span synchronous installing bridge girder erection machine and the installation method thereof, the matching construction among the supporting structure consolidation, the pier body installation and the segmental beam installation is realized.