Abstract: The paper defines the concept of generalized moment corresponding to the shear lag displacement and the relevant geometrical properties, then a finite segment element method is proposed to simplify the shear lag analysis in complex box girder with varied cross-section. The homogeneous solution of the governing differential equation for the shear lag analysis is adopted as the displacement function. The formulas of the element stiffness matrix and equivalent nodal forces are obtained by taking the constants of integration as conversion variables. The general formula for stress is presented in which the element nodal forces are employed. The finite segment element program is developed and is used to analyze simply supported, cantilever, and continuous plexiglass box girder models. The theoretical results are in good agreement with the experimental results. The generalized moment for shear lag is numerically analyzed, which is similar to bending moment in distribution and magnitude under vertical load.