Abstract: Objectives To study the effects of initial defects and combined bending and torsion loads on the ultimate strength, residual bearing capacity and collapse characteristics of box beams under single and cyclic bending and torsion combined loads.Methods ABAQUS software was used to calculate the nonlinear finite element numerical calculation of the ultimate strength of the box girder under the combined load of bending moment, bending torsion and cyclic bending moment and cyclic bending torsion. The effects of initial defects, materials, and loads were taken into account in the study. Results The results show that the plastic collapse deformation of the box girder expands along the torsional force under a single bending and torsional load, and the ultimate strength of the box girder model using the Chaboche material model is higher than that of the box girder under a single bending moment under the cyclic moment load. Considering the welding residual stress, the maximum change range of the box girder model with the highest ultimate strength is 0.51% compared with the box girder model with the smallest ultimate strength. Compared with the box girder model with the smallest ultimate strength, the maximum change range of the five groups of cyclic amplitude box girder models without welding residual stress is 1.94%, and the ultimate strength of the box girder is