In order to analyze the distortion effect of thin-walled box girder under lane load, the distribution law of stress amplification coefficient in thin-walled box girder was studied. The distortion angle and distortion generalized bending displacement were taken as node displacements. Based on the initial parameter solution of the homogeneous equation of the distortion control differential equation, the thin-walled box girder was divided into beam segment elements, and its element stiffness matrix and equivalent node load array were derived. Using Fortran language and referring to the calculation program FRAME2 for planar frame structures, a finite element program for beam segments that can be used to analyze the distortion effect of thin-walled box girders was obtained. The program was used to analyze the variation laws of the distortion internal forces and stress amplification factors of box girders under lane loads. The results show that the distortion warpage stress of three-span continuous box girder with variable cross-section under the action of lane load has a sudden change at the place of concentrated load, where the maximum and minimum values are obtained, the longitudinal distribution of stress amplification factor is similar to the longitudinal distribution of distortion warpage normal stress, both of which are symmetrically distributed along the span, the magnification factor of normal stress at the top and bottom of the section is 1.080 and 1.180 respectively. The points with smaller bending stress should not be considered when the magnification factor is taken into account.