A duty valve drove hydraulic brake servo system of HEV(hybrid electric vehicle) power transmission test rig is studied.The brake performance depends on the responses of the pressure control.A PI controller is designed through analyzing the working characterizes of the hydraulic brake system and duty valve by using rapid control prototype(RCP) tool-dSPACE,all the model and control code are successfully download to the AutoBox,and rapid control prototype experiment shows that the benefits of developing duty valve drove hydraulic brake servo system with good performance has been achieved.