The modeling and study of three-stage electro-hydraulic servo valve are presented in this paper. According to its construction and principle, its model is built. The results of simulation and experimental test are consistent, which proves the validity of the model. Based on the study of the model, the performances of three-stage electro-hydraulic servo valve are mainly affected by pilot valve performance, spool and sleeve performance, position transducer performance and servo controller performance, which shows that there are four main key technologies. The study results have a guiding role for developing higher performance three-stage electro-hydraulic servo valve.