Abstract: In order to solve the problem of low cycle fatigue life prediction of welded structure of railway freight car body, firstly, the structural strain calculation theory of low cycle fatigue under different yield states is derived, and the program is designed based on the calculation theory. and the theoretical verification is completed through the welded joint test. Secondly, in order to further illustrate the applicability of structural strain method calculation theory of low cycle fatigue, the plane strain finite element model and structural strain theory are compared; finally, the engineering application of low cycle fatigue life analysis of express freight car is carried out by combining virtual bench with structural stress strain. The results show that the structural strain method and program based on low cycle fatigue can solve the problem of low cycle fatigue of railway freight cars when the stress of welded structure is higher than the yield strength 150 MPa, and the result of plane strain model is consistent with that of structural strain theory, but after exceeding the yield strength 150 MPa, the error increases with the increase of structural stress. The research in this paper provides a good technical support for the popularization and application of structural strain method in low cycle fatigue.