The model of extracting a skeleton of mechanism is established from an existed three-dimensional CAD product model. Using assembly features, a recomposition of assembly information is carried out. At same time, geometric primitives are defined and classified. And according to the information, kinematic pairs are distinguished and matched. Using the given methods, a prototype system extracting and drawing a skeleton of mechanism is developed. Extracting a skeleton of mechanism from digital mockup has been realized primarily, which can deal with rotation pair, sliding pair, gear pair and multi-pair components consisting of these kinematic pairs. Finally, the paper demonstrates the feasibility of extracting a skeleton of mechanism by an application instances.