In order to investigate the influence of performance parameters of the recirculatingball hydraulic power steering gear on the characteristics of the automobile steering system, which used a Modelicabased unified modeling language in multifield, a mathematical model was created in detail of the steering gear including mechanical subsystems and the subsystems of hydraulic power in the simulation platform of Dymola software. The model was associated with other parts of the existing mechanical model of the vehicle steering system, and the whole simulation model of hydraulic power steering system was constructed. The results show that the accuracy of the created model of steering gear and model of steering system is verified through on spot steering experiment with considering assistance force or not, and the actual steering characteristics of automobile can be shown. With using of this model, the influence of performance parameters of the steering gear on the vehicles handling and stability characteristics can be obtained through simulation analysis, which provides a reference to further parametric optimization of the steering gear.