Abstract: In this paper,nonlinear dynamic response under loss of coolant accident（LOCA） transient in the nuclear reactor coolant system（RCS） is investigated with ANSYS program.Many nonlinear factors,such as different structural stiffness in tension and compression,gap and plastic strain caused by main pipes’ break etc.are considered.The Secondary development of ANSYS is performed to form the customized module for implementing effective parameterized and modular modeling and LOCA nonlinear analysis.Comparison of the results calculated by ANSYS and program-specific shows that the results are generally consistent,but there are some differences locally.According to the experience of transient dynamic analysis,and there are so many non-linear factors in the reactor coolant system,the difference is acceptable.The efficiency of this engineering analysis is improved remarkably with the convenience of input and modeling,viewable layout and automatic creating of reports,when using the customized module based on ANSYS.