Nowadays, bridge cranes have a tendency to be designed for big lifting capacity, wide span with high operating speed. Several long horizontal cracks have been found in web plates of the rail girder, which is terrible threat to crane’s safety. Firstly, the local stress distribution of web plates is illustrated in a semicircle form based on the elastic-plastic contact theory. Then the cyclical variation of shear stress is analyzed due to high frequency and impulsive moving wheel loads. Finally, a fatigue crack propagation model is proposed according to the strain energy density law. It is found that the shear stress and welding residual stress give rise to the fatigue horizontal crack near welding seam.