The response-based transient stability control technology is a hot issue in real-time transient stability analysis of power system. It has such advantages as independent of modle and parameters of system, real-time detctction and less calculation. Transient instability detection method based on angular velocity-rotor angle trajectory is currently a very active branch of response-based real-time transient stability analysis. Firstly, the theory of this method is introduced, then the problems in application of the method to multi-machine power systems are analyzed and discussed.The simulation results on IEEE-39 bus system show that when this method is applied to multi-machine power system, because of incorrect generator-clustering and the time-varying parameter of the equivalent system, this method will produce incorrect stability detection.