Because of the advantages of small volume and convenient installation,the hydraulic retarder is used more and more widely in vehicle. A small hydraulic retarder and its hydraulic control system were designed. The control system is simple, compact and easy to operate and suitable for small vehicles. The rotary valve was adopted to control the inlet and outlet area of the retarder then to control the braking torque of the retarder.The rotary valve has the advantages of simple structure, quick response and small hydrodynamic force. The hydraulic retarder and its control system can provide stable auxiliary power for small vehicles, and the precise control of multiposition can be realized.