To analyze power system with centralized small and medium hydropower generator group accurately, this paper proposes a method to build a dynamic equivalent model of small and medium hydropower generator group based on phasor measurement information. Some procedures, such as inputting the phasor measurement information which exists in interconnection line after a disturbance, solving the generator dynamic equations based on 4-order Runge-Kutta method, and using modified PSO algorithm to update each parameter, are used in this method based on estimate equivalent method. For validation purposes, the proposed method is applied to the centralized small and medium hydropower generator group in Sichuan province. Results show that the reduced system retains the dynamic performance of the original system with good accuracy. Further simulations are made to compare with small and medium hydropower generator group which represent as a negative load. Results prove the superiority and practicability of the dynamic equivalent method.