Abstract: [Objectives] In order to research the effect of welding initial deformation and residual stress on plastic distribution and ultimate strength of a box girder under cyclic bending moments. [Methods] Nonlinear finite element numerical simulations on ultimate strength of a box girder under various cyclic bending moment modes were carried out by ABAQUS. Welding initial deformation, residual stress, hardening of the material, and Bauschinger effect were considered in the research. [Results] Research shows that under unidirectional cyclic bending moments, the stiffness and ultimate strength of box girders decrease continuously as the number of cycle increases. The plastic deformation extends from the upper deck of box girders to the side plate region. In addition, the plastic distribution region of box girders with welding initial deformation and residual stress is wider in the ultimate state. Under bidirectional cyclic bending moments, when the number of cycle is the same, the residual ultimate strength of box girder decreases as the amplitude of the cyclic load increases. [Conclusions] The obtained results are of reference value for further research on ultimate strength of box girders under cyclic bending moments.