Abstract: For the purpose of making the fatigue life of the weld line in a welded structure under random loads predictable, a series of theoretical deduction is performed to demonstrate the existence of the linear transfer relationship between the load and equivalent structural stress. Then a new fatigue life probability prediction method for welded structures is proposed based on the random vibration theory and the master S-N curve method. The new method can solve the problem of vibration fatigue life prediction for welded structures under multi-random loads. It has the ability to take the effect of load frequency into consideration while calculating the fatigue life of a weld line, which was proved by comparison analysis of T-joint. Actually measured acceleration spectrums on a track were apply in the vibration fatigue life prediction model for a railway container flat car. The calculation results indicate that the predicted results coincide with actual situation. The convenience of the proposed method in engineering application is proved.