According to the relationships among the flexural shear stress, shear deformation and sectional longitudinal displacement of

thin-walled box girder, the distribution function of longitudinal displacement around the cross-sectional perimeter of the girder was

derived. Based on this function, the optimized shear-lag warping generalized coordinate function for the full cross section of box

girder was proposed. Therefore, according to the energy variation principle, the flexural shear lag differential equation system, which

considers the shear deformation of flanges and webs with single shear-lag generalized displacement parameter, was established. The

equation system has the same form as that of existing ones, and it unifies the shear lag analysis of flange and the bending-warping

analysis of web. Based on the analysis of simply supported beam and cantilever beam under the uniform load, it was found that the shear

lag coefficient and deflection using the present algorithm agree well with those using the block element model. Compared with the

existing algorithms without considering the shear deformation of web, the present algorithm may lead to smaller errors in both shear

coefficients and deflections.