To describe the dynamic characteristics of induction motor loads in power systems more precisely， a dynamic equivalent method for motors based on dynamic similarity and buffer zone is proposed. First， a quantitative index of dynamic similarity is defined according to the difference of mechanical and electrical characteristics between two motors， then the buffer zone is obtained according to the electrical connections between buses in the external system and the boundary. Moreover， motors and static constant impedance， constant current， constant power（ZIP）of different load buses are moved to the equivalent load node through an ideal transformer with complex ratio. Parameters of the equivalent motor are determined based on the group characteristics in the startup and no-load conditions. Finally， this algorithm is applied to the IEEE 10-generator and 39-bus test system. The simulation results show that the dynamic influence of the aggregated motors on the study system is retained with satisfactory accuracy. The proposed method has a better solution of both transient and static variables than the classical weighted equivalent method.