Abstract: In this paper, based on the analysis of the characters of continuous prestressed concrete Box-girder bridges and the reasons of cracking, cracks in them are classified into 9 types. Then a U.L formula of space analysis for variable cross-section beams is deduced, based on the theory of continuous medium, which can generally consider the secondary stress caused by distortion, lateral load distribution and warping. A program is also edited to do the liner & nonlinear analysis for concrete multi-cell box beams conveniently, the tension and the cracking regions deduced by which is consistent with that of survey on Huangpenxi Bridge.