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, and they continue to crack after reinforcement. In order to get the exact cause of cracks in the box girder, a three-dimensional solid finite element model simulating the construction process is established based on ANSYS software. 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 thinning, the reduction of vertical prestressed reinforcement and the spatial effect can easily lead to cracking in the chamfering position of web and floor and the single box single chamber section is not suitable for 19.3 wide bridge.