Since there is no body load data such as core disk and side bearing, the load spectrum of freight car body fatigue simulation analysis is usually converted from the standard load spectrum of similar models, and the calculated results are different from the actual life of the car body. In order to obtain the load spectrum of the actual car body circuit and improve the calculation accuracy of fatigue life, a virtual car body fatigue test bench was introduced. The virtual car body fatigue test rig adopted the driving load data simulated by the real car body circuit to ensure the accuracy of the simulation input. After the virtual test calculation, the calculated load data of body core disk, side bearing and coupler were extracted, and the load spectrum of typical working conditions was constructed according to the damage equivalence principle. The load spectrum was then combined with the main S-N curve method to calculate the life of the welding of the car body. The results show that the fatigue evaluation based on the load spectrum obtained indirectly by the virtual car body bench test and the main S-N curve method solves the problems of inaccurate car body load input and grid sensitivity by the nominal stress method, which is more practical than the traditional car body fatigue evaluation.