Hydraulic power steering systems, which can provide large steering assist force, are widely used for heavy-duty commercial vehicles. For the research of stability control system for commercial vehicles, the vehicle's combined simulation model, including IPS (integral power steering) system and vehicle body, was constructed. Then the linearization and order-reduction of simulation model were done for model based controller design. Comparing with experimental results, it was found that the established simulation model can adequately represent the vehicle dynamic behavior, and the linear low order model was also verified by comparison of eigenvalues and frequency domain responses. Finally, a yaw rate control system was designed based on the linear models and tested on the combined simulation model, which shows the validity of the simulation model and linearized low order model.