The core structure of high flux experimental reactors is very complicated, and the method of characteristics is one of the effective methods to realize its high-fidelity neutron transport calculation. Refined modeling and division of coarse meshes and fine meshes for the core structure are the key to the realization of the MOC method. Under the conventional regular coarse mesh division, especially in the control drum region, it is difficult to find a correspondence between coarse meshes and fine meshes. In this paper, we use the modeling method of constructive solid geometry and adopt a new mesh division method based on the irregular coarse mesh division, and develop the geometric module and the MOC calculation module of the HNET-HFR program. By comparing with conventional meshing method, this irregular coarse mesh division has a smaller memory consumption and faster modelling time, while the MOC tracing takes less time. The computational results show that the new mesh division method is correct and has higher accuracy.