Abstract: By mean of potential variational principles, the stiffness matrix of thin-walled curved box girders in considering the shear lag effect of flange’s stress and material nonlinearity is obtained. The elastic and plastic problem of thin-walled curved box girders is solved with the spline finite point method and the section internal force plastics coefficient method. The research shows that the nonlinear property of the moment shear lag coefficient is more obvious than that of the displacement and torsion; when the load reaches certain values, the distribution of flange’s stresses in the section of box girders become more and more uniform with the increasing of the load. The proposed method is simple, practical and could be also used to calculate thin-walled curved box girders with the varying section and varying curvature.