Abstract: The ultimate flexural capacity of the external prestressed composite box girder with corrugated steel webs is investigated. For this purpose, based on the quasi-plane assumption of flexural strain distribution and axial force-bending moment-curvature relationship (N-M-φ) of sections, a nonlinear full-range-analysis program is developed to perform the flexural behavior analysis. The program considers effects of material nonlinearity, inconsistent strain between beam and external tendons as well as second-order effects of external tendons. Furthermore, an experiment is conducted on a simply supported beam with corrugated webs, and the numerical results obtained by the program are confirmed. Moreover, it is shown that (1) the whole process of the flexural rupture can be divided into three stages: elastic stage, crack stage and plastic stage. (2) the second-order effect of external tendons plays an important role on the ultimate flexural capacity of the beam, and the mid-span diaphragm can greatly reduce the second-order effect of external tendons.