Based on the research of previous models of CAD systems, the component-based model is presented in this paper, aiming at the openness, integration and efficiency of CAD systems. Functions of components and communication among them are discussed in detail. The model has a double-bus architecture, is better in openness than the previous models, is proved to be a solid base for the modelling and implementation of efficient CAD systems with clear architecture. In addition, components can be used as the base of software reuse.