In order to study the free vibration characteristics of box girder under the joint influence of longitudinal static equilibrium and shear lag effect, the free vibration differential equation and boundary conditions of simply supported beam are derived according to the energy variation principle.TheMatlab software is used to solve the problem and compare with the Elementary beam theory, the ANSYS finite element method and the results obtained considering the shear lag effect.The results show that the calculation results are consistent with the finite element calculation results and the accuracy is reliable. It is also found that the longitudinal additional axial force will reduce the relative stiffness of the simply supported box girder, which will lead to the decrease of the self-vibration frequency.With the increase of mode order and the decrease of span-width ratio, the effect of shear hysteresis on the frequency of self-vibration increases gradually.