A bridge is (25+31.6+29.8+29.8)m prestressed continuous box girder bridge,and it has been founded that number of cracks appeared at the bottom of the girder after the main girder formwork was removed. In order to find out the causes of the cracks, make clear the current stress state and function degradation of the box girder, analyze the impact of the cracks on the bridge, we tested and evaluated the main girder, and used the finite element software to check the structure.The test results show that: the main appearance disease is the transverse cracks under the cavity of the bottom plate box chamber.Based on the analysis of bridge detection and calculation results, the causes of box girder floor cracks are concrete shrinkage caused by improper construction measures in the construction process, not insufficient structural compressive stress reserve caused by prestress loss. This kind of cracks will not impact on the structural performance of box girder, but will affect the durability of box girder.