Abstract: An element for thin-walled box beams is proposed for the flexural-torsional analysis of skewly supported continuous box girder considering the constraint condition in the supporting coordinate system. The element has ten degrees of freedom and can conveniently simulate shear lag effect and torsional-warping deformation of skewly supported continuous box girder. The element stiffness matrix is derived by taking the homogeneous solutions of the governing differential equations for flexural shear lag and restraint torsion as the element displacement functions. The shear lag warping displacement function for the twin-cell box section is established through the axial equilibrium condition of the shear lag warping stress. General formulas for the geometrical properties of cross section concerning the shear lag warping are presented. The program SSCBA is developed and used to analyze a skewly supported three-span continuous box girder model with twin-cell cross section. The calculated results are in good agreement with the experimental results and the calculated results from shell element of ANSYS. It is shown that the warping deformations of shear lag and restraint torsion affect remarkably the stress distribution in a skewly supported continuous box girder under eccentric load at the mid-span.