In order to have good alterable assist characteristics with speed on vehicle electrical controlled hydraulic power steering system (ECHPS), the opening control strategy for bypass flow proportional valve of ECHPS was studied. The basic principle and structural form of bypass flow controlled ECHPS were analyzed, and the dynamics and control models of proportional valve were established. Aimed at the requirements of control the bypass flow of ECHPS on dynamic multi-mode conditions accurately and stably, a fuzzy PID control strategy was proposed. The fuzzy reasoning was applied to self-tuning of the PID parameters to compensate the error caused by the nonlinearity and time-variation of the control system. The results of simulation show that, the bypass flow of ECHPS and the opening response of proportional valve at low, medium and high speed conditions were comply with the request for alterable assist characteristics with speed. As compared with the conventional PID control, the designed fuzzy PID control has advantages of less overshoot, higher steady-state accuracy and rapid response, which can realize the opening control of proportional valve accurately and stably.