In rapid tooling, guaranteeing the dimensional accuracy of the casting die is a key technical problem for RT technology. The boundary model between casting and mold directly influences the accuracy of the numerical simulation of solidification for rapid tooling. Three different boundary models are adopted to analyzed the solidification process of a typical part and the simulation results are compared with experimental results. The comparison results indicate that the contact boundary model used in this paper is more accurate for numerical simulation of dimensional accuracy for solidification processed in rapid tooling. The contact boundary model assumes that both the mold and the casting are thermally elastic-plastic deformable bodies and analyzes the boundary problem using the method of contact elements. This contact boundary model is used for a coupled thermo-mechanical analysis of the die of an automobile deck part. The simulation result matches well with the practical casting, which further demonstrates that the contact boundary model used in this paper is accurate and feasible.