As the most important part of the power system load, the dynamic characteristics of motors are very important for power system simulation. The calculation is very large if all motors are modeled respectively. In order to reduce the amount of calculations, a dynamic equivalent modeling method of motors based on the improved hierarchical clustering algorithm is proposed in this paper. The two state parameters, the load rate and the product of inertia time constant and rotor resistance, are taken as clustering indexes. The improved hierarchical clustering algorithm is used to cluster dynamically the motors. And with the pseudo t2 statistics, it can obtain the best clustering results. Then it establishes the dynamic multi-machine equivalent model after each motor group equivalent. With the example system, the simulation results show that the proposed dynamic equivalent method can keep the dynamic characteristics of models and improve the equivalent accuracy.