High speed on-off valve is applied widely in vehicle control systems. When high speed on-off valve is controlled by Pulse Width Modulation (PWM) of high frequency, the valve core can float at a certain position which is adjusted by changing the duty ratio within a certain effective range. Then the high speed on-off valve can control the flow and pressure linearly like proportional valve. Thus it is essential to extend the effective range of duty ratio to improve the linear control performance of high speed on-off valve.In this paper, the high speed on-off valve of the automotive Electronic Stability Program (ESP) is the focus, and its flow force is analyzed in detail to get the effects of hydraulic parameters on the valve performance. The mathematic model of the high speed on-off valve is derived. Then the valve structural parameters are optimized according to the Genetic Algorithm(GA), offering the theoretical references for extending the effective duty ratio of PWM. Besides, the simulation model of high speed on-off valve is established in MATLAB/Simulink, and then embedded in the Hydraulic Control Unit (HCU) simulation model established in AMESim, resulting in the co-simulation model of hydraulic actuator. Finally the simulation model is verified by the hardware-in-loop tests. Based on theoretical analysis, simulations and tests, it is critical to increase the range of flow force for extending the effective range of duty ratio. The pressure difference, the valve seat angle and the throttle diameter of the high speed on-off valve have great effects on the flow force. At last the effects of structural parameters of the high speed on-off valve on extending the range of the linear control are proposed, serving as references for improving the linear control performance of high speed on-off valve.