Abstract: Based on the flexural theory for box girders, the deflection function of the composite box girder with steel bottom plate and corrugated steel web plate is proved to be quadratic.The shear lag effect of corrugated steel web composite box girders is investigated through the variational principle of combined energy.The experimental results and finite element model have proved to be more realistic and reliable.Due to the difference between the two-dimensional stress characteristics of the roof and the unconstrained free end of the cantilever plate, there is an error between the theoretical value and the finite element value.To simulate realistic experimental conditions, the necessary adjustments was made by introducing correction coefficient.The results shown that the theoretical value is more consistent with the finite element value when the correction coefficient is 1.4.The axial equilibrium conditions were discussed and the results shown thatwhen the width to span ratio of corrugated steel web composite box girder is less than 0.5 or the plate width ratio is less than 0.75,the axial balance condition should be considered.