An analytical solution method was proposed to solve the problem of shear lag. A new shear lag warping function for the shear distribution of different cosine was constructed based on the solution of the governing equations. The trigonometric series were used to expand the shear distribution function under any given external loads, and the normal stress corresponding to each shear component was obtained separately. Finally, the superposition of normal stress was obtained and the distributions of shear lag were calculated. The energy variation method was used to derive the solution formulas based on the arbitrary warping displacement functions of shear lag effect, and a general solution procedure was developed. A rectangular simply supported box beam without cantilever plate under concentrated load and a box beam with cantilever plate under distributed load were analyzed. The calculation results show that the solution method proposed in this study could adapt well with different types of loads compared with the existing methods, and it could be effectively used for shear lag analysis of box beams subjected to arbitrary loads.