Research conclusions: As the crack generated among end-span, side-span and mid-span(1/4～3/4L) of the web plate weaken the rigidity and strength of the cross-section, so the applied reinforcement method can improve the rigidity and strength of the cross-section with feasibility and money saving. Through making calculation and comparison, the amplitude variation of the internal force before and after cracking is small. Because of the loss of the vertical prestress, the shear force of the cross-section is close to or lower than the most unfavorable design value of shear force, resulting in large incipient fault of the bridge structure during operation , so it is necessary to improve the strength against shear force in time due to the loss of the vertical prestress. The inversion method is used to analyze the factors of the disease. With the sticking steel plate and thickening web plate of the girder, the loss of the rigidity and strength of the girder cross-section can be compensated