The servo valve orifice area of negative flow control mechanism in the variabledisplacement pump was analyzed. The mathematical model of the negative flow control mechanism of variabledisplacement 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 variabledisplacement pump was analyzed. Valuable reference is provided for the design of servo valve orifice.