Hydraulic power steering gear supplies assistant power by liquid stream. The stream will be changed greatly for the flow field formed by the rotary valve will vary during steering. And the change of the stream in this narrow field will (be big enough to) cause the fluctuation of the assistance power and noise of the steering gear. 3-D meshes of the flow field between the sleeve and the rotor of the valve with different structure parameters such as entry caliber and groove depth were set up and a general CFD code-Fluent was used to analyze the stream performance. The results including pressure and gas-phase's volume fraction were analyzed under certain operation flow rate. It was found that the entry caliber did not affect the operation pressure of the valve under the same flow rate but affected the gas-phase's volume fraction of the flow field, and so did the groove depth of the valve' sleeve and rotor. Many researches have pointed that the noise of steering valve is almost cavitation noise, that is, the gas-phase in the flow field has great correlation with rotary valve noise. Based on these analyses of the stream in the rotary steering valve, this paper gave suggestion that increasing entry caliber of the rotary valve and choosing appropriate groove depth will reduce the valve noise. The stream will be changed greatly for the flow field formed by the rotary valve will vary during steering. And the change of the stream in this narrow field will (be big enough to) cause the fluctuation of the assistance power and noise of the steering gear. 3-D meshes of the flow field between the sleeve and the rotor of the valve with different structure parameters such as entry caliber and groove depth were set up and a general CFD code-Fluent was was used to analyze the stream performance. The results including pressure and gas-phase's volume fraction were analyzed under certain operation flow rate. It was found that the entry caliber did not affect the operation pressure of the valve under the same flow rate but affected the gas-phase's volume fraction of the flow field, and so did the groove depth of the valve 'sleeve and rotor. Many researches have pointed that the noise of steering valve is almost cavitation noise, that is, the gas-phase in the flow field has great correlation with rotary valve noise. Based on these analyzes of the stream in the rotary steering valve, this paper gave suggestion that increasing entry caliber of the rotary valve and select appropriate groove depth will reduce the valve noise.