Abstract: Introduces the application of a pump controlled motor system, and expounds the flow control principle of axial-piston pump under time-varying input speed. Torque characteristics on swash plate of the axial piston pump is studied. Mathematic model for the flow control is derived for the pump running under a time-varying input speed. The control scheme which is a combination of pump input speed product compensation and closed loop PI controller is designed to constrain the speed disturbance. A simulation model of pump controlled motor system is established on the simulation software EASY5. Results of simulation showed that the displacement adjusting time of the pump agreed with the manufacturer's curve. The pump kept an invariable output flow under the long-range disturbance of time-varying input speed and (load pressure.)