Abstract: The conservative analysis method in the analysis of loss of coolant accident(LOCA) is not conducive to improving the economic efficiency of nuclear power plants. In order to satisfy the evaluation requirements for LOCA of nuclear power plants in 10CFR50 Annex K, the model was modified to meet the evaluation requirements for LOCA while increasing the design margin based on the best estimation code of RELAP5. Because Appendix K involves large models, this paper mainly studies the method of modifying and verifying the LOCA code model. The revision of the critical flow model of RELAP5 code was carried out. A conservative Moody two-phase critical flow model was added. At the same time, the Zaloudek model for critical flow calculation was added. The separate effect tests of Marviken and Edward’s pipe and the integral effect test of Bethsy were used to verify the code. The results show that the added model is reliable enough to simulate the critical flow phenomenon in the discharge process.