The longitudinal cracks in the bottom slab of box girder have been found in lots of prestressed concrete continuous girder bridges in China, and the disease has been common by now. In this paper, a review of the causes of longitudinal crack is given. Then an analysis model is established by MIDAS/Civil 2006 to analyze the causes of longitudinal cracks in the bottom slab of an extra-large bridge. In calculations, dead load, vehicle load, prestressed load and temperature load etc are considered, and the distributions of transverse stress are obtained under three load combinations. The results show that the downward force caused by prestressed cables and the eccentric load under motor vehicles are the main factors which cause the appearance of these cracks. Finally, suggestions for the design and construction of such bridges are proposed to prevent the cracks, which can be referenced by relevant people.