Abstract: Model data representation is the foundation of the CAD geometry engine. The Boundary Representation (B-Rep) and Constructive Solid Geometry (CSG) representations are two commonly used model data representation methods. With the development of hybrid modeling, B-Rep and CSG hybrid modeling has been applied to large-scale models such as ships and the conversion between B-Rep and CSG is a key function. For the B-Rep to CSG conversion of primitives, a process was designed to parse the geometry structures in a B-Rep entity and combine them into a CSG primitive based on the topological structures. For conversion of complex entities, faces in a B-Rep entity are divided into different regions based on its convexity, and the boundary of each region is a cutting loop. Then a cutting loop recognition algorithm is proposed. Based on the cutting loop recognition results, a binary decomposition tree is designed for entity decomposition and CSG tree generation process. Fifteen types of basic entities and four complex models are used to make several B-Rep entities, and the conversion errors and time costs are measured. Experimental results show that the proposed method can achieve B-Rep to CSG conversion within an error range of 0.05% of the normalized model size, and has supported the model light-weighting function of the component sample modeling platform of the Ship Product Design System (SPD).