Abstract: In order to analyze the stress amplification factor of thin-walled box girders subjected to eccentric lane load considering torsion and distortion, based on the initial parameter solutions to the governed differential equations for constrained torsion and distortion, the analytic solution to the warping internal force of simply supported box girders was derived, and the relevant warping stress was further obtained.A simply supported thin-walled box girder was adopted to analyze the stress amplification factor relative to bending stress by considering torsion and distortion warping stress.The results show that the mid-span normal stress amplification factor at the intersection of web and bottom plates reaches 1.27,and the shear stress amplification factor for web plate at the horizontal centroid axis reaches 2.21.The calculation results for distortion and warping stress indicate that the distortion warping normal stress is about 5 times to the torsion warping stress at the intersection of web and bottom plates in mid-span cross section.The distortion warping shear stress of thin-walled box girders is all relatively large in the bottom plate, web and cantilever plate.