Abstract: A support coordinate system is established for thin-walled box girders in which the compatibility of displacements and balance of forces are met. The element stiffness matrix and equivalent joint forces are derived for thin-walled box girders with arbitrary asymmetric cross-sections to handle box girder under tension (or compression), bending, torsion and warping. In addition, a general procedure is proposed to treat the complicated constraints of thin-walled box girders by adding an assumed spring element with linear stiffness, rotational stiffness and torsion-warping stiffness. It is shown that, when the effect of warping on rotation angle is neglected, tension (or compression) is only coupled with horizontal bending and vertical bending instead of constrained torsion. The effectiveness of the analysis method is validated through numerical examplles.