In view of the excellent features of the jet deflector servo valve with comparison of other valve products but lack of its domestic refined, the optimal design of its key structure, amplification stage, is proposed. The CAD software is used to make model for the amplification stage flow field of the jet deflector servo valve, and several models are made for the structure size which need to be designed. Then these models are properly divided into mesh in order to analyze its flow field so that we can get the distribution condition of the flow pressure and velocity on different structure size and moving situation. At last through analyzing the experiment data and surface fitting, the analyzing result and working surface could be used to guide the parameter selection by selecting good working point, namely the parameter optimization.