Abstract: The box girder's wing plates(including the roof, the cantilever plate and the bottom floor) are divided into parts of different generalized longitudinal displacements of shear lag. Taking a parabola as the transverse distribution form for each wing plate and introducing lateral position parameters 'η', the shear lag effect is analyzed for cases with varying lateral loading locations. A control differential equation is built for the shear lag effect of the box beam and it is solved for the simply supported beam and the cantilever beam under uniform load by using the energy variation principle. Examples show that the varying lateral loading locations affect the positive and negative shear lag characteristics of the loading wing plate and the magnitude of the stress in the non-load plate. The lateral framing effect makes much greater contribution than the shear deformation for the longitudinal stress. The result is consistent with the analytic result of the block finite element model, which shows that the algorithm can accurately analyze the shear lag effect of the box girder for cases with varying lateral loading locations.