Many parallel robots as machine tools have been designed and manufactured, while only a few are commercialized in actual applications. One of the important reasons is that the large divide between theoretical accuracy and actual accuracy. Although some complete static stiffness modeling methods and some overall static stiffness indexes for parallel robots have been proposed, but there is relatively little research on the dynamic stiffness performance, which is very important to guarantee the best machining accuracy of parallel robots as machine tools. Considering the processing requirements of the aspheric lens, a machine tool based on a 3PUS + 1PU parallel robot is designed. Taking the 3PUS + 1PU parallel robot with a passive chain and few degrees of freedom as the implementation object, the kinematics and dynamics analysis are presented respectively, and the dynamic stiffness model is derived based on the Kane’s equation and screw theory. Comparing with the distribution of dynamic stiffness in the workspace of the proposed mechanism, the proposed method enriches the research of the dynamic stiffness modelling of parallel robots as machine tools, and the result can also provide reference for structural parameter design and vibration control optimization.