4N?m。The rotary valve parameters have significant influences on the assist effort characteristics of the flow control type electronic controlled hydraulic power steering (ECHPS) system. The calculating procedures of the equivalent flow area and assist effort characteristics of a flow control type ECHPS system were analyzed using the principles of hydraulic resistance networks. The mathematical relations between the orifice area of the rotary valve and the configurable design parameters (the metering edge width, the metering edge eccentric distance, the flange width and the slot length of the input shaft) were deducted. Subsequently, the optimum parameters of the rotary valve were obtained by optimization design technique. The values of these parameters were optimized from 0.9mm, 10mm, 5.76mm and 11mm to 1.115mm, 4.626mm, 5.652mm and 4.499mm. The numerical simulation results show that the alterable assist effort range is increased from 1.2 to 3, and the maximal steering wheel force is 4N?m in low speed.