Abstract: Considering that the conventional typical geometric processing method used in high-fidelity neutronics code NECP-X can hardly handle the complex small reactor, hexagonal fast reactor and so on. CSG（Constructive Solid Geometry） method, which models complicated geometry by the Boolean operation of some simple surfaces, has many advantages, such as geometry flexibility, small memory requirement and easy to be extended. Therefore, the CSG method is applied to NECP-X in this paper, which extends the geometric processing capability. The resonance and transport methods are updated to use the CSG method, and the auto-mesh function is developed to generate meshes with only several parameters. The information of characteristic segments is obtained by the intersection of lines and the constructed geometries. The numerical results show that different reactor cores can be modeled in detail and calculated with the one-step transport method, and the computational results agree well with MC（Monte Carlo） code.