In the past, when the solidification of castings has been simulated, the mould has not been considered or it has been treated as rigid while the boundary model between casting and mould has been considered. In qualitative analysis or semi-quantitative analysis, these assumptions may be reasonable. However, for numerical simulation of the dimensional accuracy of precision castings, which is a quantitative problem, these simplifications are no longer accurate. The boundary model between casting and mould directly influences the accuracy of numerical simulation of solidification for precision castings. This paper discusses in detail the boundary model between casting and mould and its influence on the accuracy of numerical simulation for precision castings. Three different boundary models are adopted to analyse the solidification process of a typical part, and the simulation results are compared with experimental results. The first boundary model assumes that the mould gives way. The second boundary model assumes that the mould is rigid and the casting is a thermoelastic-plastic deformable body. The third boundary model assumes that both the mould and the casting are elastic-plastic deformable bodies. The comparison between simulation and experimental results indicates that adopting the third kind of contact boundary model and analysing the mould and die both as deformable bodies would be more accurate for numerical simulation of dimensional accuracy in the solidification of precision castings. This contact boundary model, assuming both the mould and the casting to be elastic-plastic deformable bodies, is applied to the coupled thermomechanical analysis of the die of an automobile deck part. The simulation result also matches the practical casting quite well. This demonstrates that the contact boundary model adopted in this paper is accurate and feasible.