The invention discloses an external prestress-based seismic beam falling prevention limiting structure. The external prestress-based seismic beam falling prevention limiting structure comprises main beams and a limiting pier at the ends of the main beams; separators and stoppers are arranged above the limiting pie, wherein the separators are arranged in the transverse direction of the main beams and separate every two longitudinally connected main beams and the stoppers are arranged at intervals on the side surfaces of the corresponding separators to form limiting grooves for embedding main beam webs; every main beam web and the corresponding stoppers on both sides are tensioned through anchor units; the main beams and the limiting piers are correspondingly provided with limiting structures. Through embedment and tension, transverse and longitudinal restriction and deformation energy absorption of the main beams on the limiting piers can be achieved, so that beam falling of the main beams in the transverse and longitudinal directions during earthquakes can be effectively avoided, further a bridge can continue to be put into service after earthquakes happen to ensure smoothness, and personal injuries and property losses caused by earthquakes can be reduced or even eliminated.