Abstract: In order to further simplify the calculation method of box girder bending deflection, this article used the energy variational principle and the Timoshenko beam theory to derive a formula for calculating box girder bending deflection through considering full section shear deformation. Based on the shear lag control differential equation and the conversion method of effective flange width in current highway bridge regulations, a simplified calculation method for box girder deflection through considering full section shear deformation was proposed. The analysis of simple supported and continuous box girder examples shows that the theoretical deflection and simplified method results are in good agreement with the ANSYS numerical solution, and the maximum difference ratio is basically within 5%, verifying the correctness and applicability of the proposed simplified calculation method.