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Hydraulic power steering systems, which can provide large steering assist force, are widely used for heavy-duty commercial vehicles to enhance the driving comfort. Such systems have great influences on vehicle controllability, steering stability and safety. However, interpolated table from experiment data was used as the assist force to study the vehicle dynamics in many previous works, so the transient behavior of the power steering systems due to time delays, non-linear dynamic characteristic, external disturbance and parameter uncertainty of the hydraulic components was neglected. This paper presents a detailed hydraulic power steering model consisting of a steering cylinder, a rotary valve, a pump, a pressure relief valve, and hydraulic pipes, etc., and the model is established by AMESim software. The other mechanical parts of the vehicle such as the steering system, the suspension and the power train, etc., are built by ADAMS software. A co-simulation is carried out to study the steering stability of the vehicle affected by the hydraulic power steering system. Comparisons between measured data and simulation results validate the correctness of the model, and demonstrate the effectiveness of the co-simulation method. Then some important parameters were altered to show the influences on the vehicle on-center handling performance.