The servo valve orifice area of negative flow control mechanism in the variabledisplacement pump was analyzed. The mathematical model of the negative flow control mechanism of variabledisplacement pumps was established, and the simulation was carried out using SIMULINK. The influence of dimension parameters of the valve orifice on the dynamic characteristic of negative flow controlled variabledisplacement pump was analyzed. Valuable reference is provided for the design of servo valve orifice.