When analyzing the stability of power system, it is not convenient for the analyst to know the dynamic performance of the generating units group as a whole if a detailed model is used. It could be much more convenient if a dynamic equivalent model is used instead. This paper presents a method to build a dynamic equivalent model of small hydropower generating units. Some procedures, such as aggregation of the parameters of the generators and the abbreviation model of the network, are improved to make it more applicable to build the equivalent model. For validation purposes, the proposed method is applied to a test IEEE system of 10 generators and 39 buses and the small hydropower generating units in Enshi, Hubei province. Results show that the reduced system retains the dynamic performance of the original system with good accuracy. Further simulations are made to analyze the effect of the equivalent load model, which lays a foundation for dynamic equivalent modeling.