A novel fast method for transient stability prediction is proposed based on phasor measurement unit（PMU） measurements and perturbed trajectories fitting of rotor angle. In this method， the system is simulated off-line and rotor angle trajectories are recorded under different disturbances， and the characteristic of trajectories are extracted by cluster analysis. The perturbed trajectory standard pattern database（PTSPD） is thus obtained. The Euclidean distance between the real-time measured rotor angle trajectories and the central trajectory of one mode in PTSPD are calculated， and the rotor angle trajectories are forecasted by using a modified expression， thus transient stability is predicted quickly. Taking the New England test system for example， the effectiveness and advanced property of the presented method are proved.