The problem for the low speed characteristics of the hydraulic power system with variable speed was studied by using AMESim and experimental verification method from the mechanical characteristics of induction motor and the efficiency of the piston pump.The results show the hydraulic power system of variable speed is low because of small maximum output torque of drive motor, weak mechanical properties and the low efficiency of the piston pump, the flow instability at low speed running state, which provide reference for improving the low speed characteristics of hydraulic power system with variable speed.