This paper studied the problem of low precision for variable pump displacement control method of closed hydraulic circuit. The linear relationship between the duty cycle signal and displacement of pump was derived based on the pulse width modulation（PWM） control of the servo valve for the pump control system. The simulation model of the pump control system was built on the AMESim software platform，and the model was verifiedby comparing with the test data. Based on the pump control system model，the PID feedback control，feedforward + feedback control， and three-step control of pump displacement were proposed，and an analysis was performed. Simulation result shows that the three-step control is superior to PID feedback control in dynamic response and rapid stability. When compared with the feedforward + feedback control，the dynamic response error of the pump displacement controlled by the three-step method is reduced by 35.5%.