Abstract: A 3D model pretreatment method was proposed for MCNP calculation program, including model recognition and simplification algorithm, the cavity generation algorithm, and the efficient conversion algorithm from CAD models to half space CSG models. And a 3D model pretreatment system was developed for MCNP calculation program to make the CAD model be transformed into half space CSG model rapidly. Finally a practical example was arranged to validate the developed system. The results show that, the proposed method can quickly transform complex CAD models into half space CSG models, providing an efficient 3D model pretreatment system for MCNP calculation program. It can improve the efficiency of the MCNP input file construction.