Abstract: In order to comprehensively analyze the stress amplification effect of eccentric load on thin-walled box girders, a calculation method of stress amplification coefficients considering distortion and the coupling deformation of distortion and torsion was proposed.Based on the deformation assumption of generalized coordinate method and the principle of virtual displacement, the governed differential equations of composite deformation were derived.The initial parameter solution of the deformation state vector and the calculation formulas of stress amplification coefficients were given.The correctness of the theoretical method was verified using the example of a simply supported steel box girder.The result shows that when the simply supported steel box girder is subjected to an eccentric lane load, the normal stress amplification coefficient of the top plate and the shear stress amplification coefficient of the web first reduce and then increase along the longitudinal direction from the support to the mid-span.The amplification effect of the coupled deformation stress generated by the coupling between torsion and distortion on the bending stress is most pronounced in the mid-span.In the mid-span, the amplification coefficient of shear stress at the midpoint of the web is 2.00.On the top plate, the amplification effect of the warping normal stress on the bending normal stress is positively correlated with the distance from the calculated point to the midpoint of the top plate.The eccentric load effect analysis of thin-walled box girders should consider the stress amplification effect of distortion and the coupling effect of distortion and torsion cannot yet be ignored.