Abstract: To make CSG models more readable, the paper improves B-Rep to CSG conversion algorithm by utilizing face shell shrinking technology.B-Rep to CSG conversion consists of generation of primitives and construction of CSG tree.And the face shell shrinking based B-Rep decomposition algorithm is exploited to generate suitable primitives.In order to effectively construct the CSG tree, volume relation graph (VRG) is firstly used to represent the relation among different primitives;and then an improved Stoer-Wagner mini-cut algorithm is utilized to achieve VRG to CSG conversion.The paper proves that the hard constraints "closable constraints" and "combinable constraint" can ensure the correction of the constructed CSG tree, and the soft constraints "simple decomposition constraint" and "the best balance constraint" can optimize the CSG tree.The algorithm has been implemented in self-developed multi-physics coupling analysis modeling program MCAM, and the experimental results demonstrate that the algorithm can greatly optimize the constructed CSG model and improve MCAM's time performance as well.