Abstract: For coupled calculation of Monte Carlo method and codes with deterministic method and codes, the simulated functions of unstructured mesh have been developed in the 3D general purpose N-particle transport Monte Carlo code JMCT. At present, the JMCT code supports not only the real combinatorial geometry (CSG) but also the structured/unstructured mesh. In this paper, the shielding model of Qinshan-I reactor core was chosen as testing the new function. Supermesh, a pre-processing software for automatic mesh generation of JMCT software, was used to divide the Qinshan-I model into structured mesh and unstructured mesh model based on the CSG model. The simulated results of three geometry processes were given, respectively. It can be seen from the calculation results that the error of the unstructured mesh under the three geometric descriptions of JMCT is slightly larger than that of the solid combined geometry and the structured mesh. The results of the three geometric treatments are convergent, and the maximum relative deviation from the experimental measured value is less than 14.29%, which meets the requirement of shielding for calculation accuracy (20%). The parallel efficiency of the three geometric models on the Chinese computer with 4 000 cores is more than 65%. The calculation time of solid combination geometry is the least, and time of unstructured grid is the longest.