A large number of cracks appear in the chamfering positions of webs and baseplates in the middle span of a long-span skew variable cross-section wide width continuous box girder bridge after 6 years operation. After reinforcement, the box girder bridge continued to crack. In order to get the exact cause of cracks in the box girder. Using ANSYS 15.0 analysis software to build a three-dimensional solid finite element model according to the construction process for the whole process of fine analysis. The results show that: it is not accurate to analyze the wide skew beam bridge with significant spatial effect only by using the member system beam element. The web thickness becomes thinner, the reduction of vertical prestressed reinforcement and the spatial effect can easily lead to cracking in the chamfering position of web and floor; 19.3m wide bridge should not adopt single box single chamber section.