In order to analyze the state of shear lag effect under different generalized displacements, the maximum shear angle difference of the flange plate and the additional deflection generated by shear lag were selected as the generalized displacements. The energy variational method was used to derive the corresponding shear lag coefficients and stresses of the box beam under concentrated load at the mid span based on different generalized displacements. ANSYS software was introduced to establish the finite element model of the box beam.The analysis of typical simply supported box girder examples shows that the stress values and shear lag coefficients at key points of the simply supported box girder obtained based on different generalized displacements are in good agreement with ANSYS values, and the actual stress values obtained based on additional deflections are closer to ANSYS values. The shear lag coefficient calculated by selecting additional deflection at the midpoint of the top plate is 0.846, which is 38.69% higher than the shear lag coefficient calculated by selecting the maximum shear angle difference of 0.610. The shear lag coefficient calculated by selecting additional deflection at the midpoint of the bottom plate is 0.799, which is 30.98% higher than the shear lag coefficient calculated by selecting the maximum shear angle difference of 0.610.