A flat steel box girder with circular arc lower desk was analyzed by two steps with two models,a whole bridge model established by larger scale elements and a segment model established by little scale elements.The whole bridge model was analyzed firstly to determine the boundary condition for the segment model.Then the local stress distributions of the upper desk and the lower desk were studied based on the segment model with the obtained boundary condition under the dead load and the living load.It is found that shear lag effects are significant in the upper and the lower desk and that effects of the ribs on the mid-span section are larger then that on the section near the pier.Comparisons of the lower desk stress distributions between two kinds of the flat steel box girder show that stress distributions of the girder with fold line lower desk are uniform than that of the girder with circular arc lower desk.