To transform CAD model into virtual assembly model, model transformation was divided into two processes, namely part model transformation and assembly constraint model transformation. With secondary development of SolidWorks system, both part model and assembly constraint model were extracted form SolidWorks model. The OpenFlight database, which is a standard format for virtual reality simulation, was used to represent the hierarchical model of part. Assembly constraint model was expressed by object-oriented hierarchical model in text file. These two models were associated with each other using unique name or number. By loading part files and assembly constraint file, the assembly model was reconstructed in virtual assembly system. The experiment verified that the presented method was effective.