The output flow fluctuation of piston pump has a significant influence on the working stability and lifetime of the hydraulic systems. Based on the analysis of working principles of radial piston pump and hydraulic components, the influence rule of throttle valve opening on the output flow fluctuation of piston pump has been studied. An active control method by adjusting the throttle valve opening process to suppress the flow fluctuation was established，and the simulation experiments study was carried out. The results show that the proposed method can effectively suppress the flow fluctuation of radial piston pump.