Abstract: To improve the dynamic characteristics of actual structure of parallel machine tools, they are reduced with a viscous damping model based on dynamics, which is developed according to vibration theory. Analyzing the mechanism of damping vibration attenuation, it is found that only if the value of the ratio of the external excitatory frequency input to the natural frequency of machine tool structure exceeds a certain value, the peak values of both displacement and acceleration can be decreased simultaneously by the control of variable damping so as to control the vibration attenuation. The principle of semi-active control system to improve the dynamic stiffness of parallel machine tools is discussed, and the MRD is therefore proposed to improve their dynamic characteristics. According to the Bingham plasticity model for magnetorheological structure in combination with the features of parallel machine tools, a Hooke's universal joint was designed and made for MRD.