This paper proposes a new brake-by-wire system for intelligent/unmanned vehicles. Magnetorheological fluid clutch (MRC) with the characteristics of simple structure, fast response and large range of controllable transmission torque is employed for the brake-by-wire system to realize continuous and fast braking force output of the electro-mechanical brakes (EMBs). The proposed brake-by-wire actuator is mainly composed of an AC servo motor, a planetary gear set, a MRC and a screw set. The planetary gear set amplifies the output torque of the AC servo motor, through controlling the current in the excitation coil of the MRC to realize the real-time control of the braking force at the end of the screw set. Specifically, the structure of the MRC is designed and verified by magnetic simulation. The optimization for structural design of the MRC is carried out via Mechanical APDL and the prototype is manufactured. The mechanical properties of the MRC are calibrated, and the real-time braking force tracking experiment based on PID control is conducted and analysed.