Abstract: This paper presents a theoretical solution of a simple supported trough girder under uniformly distributed loads by using the principle of stationary total potential energy. The global bending of the trough girder, local bending and shear lag effect of the slab and torsion effect of the main-beam are considered. With comparison of FEA results, the theoretical solution is verified to be correct and applicable. Following results are found through example analysis: 1) as the height-to-width ratio or width-to-span ratio increases, the contribution of the local bending to the longitudinal stress increases; 2) shear lag effect is obvious in the example, and the width-to-span ratio has great influence on the effective width ratio of the slab; 3) whether to consider torsion effect of the main-beam has little influence on the final results.