Abstract: On the basis of the rigid perimeter assumption of thin-walled curved box-girders, considering the shear lag effect, this paper establishes the flexural-torsional differential equations and the corresponding boundary conditions by the energy variation method, where the effect of secondary shear flow on the warping displacement and the self-equilibrium condition of the shear lag warping stress are also taken into account. The derived differential equations are solved using the Galerkin’s numerical method. The general formulas of the shear lag warping geometrical properties are given for the single cell trapezoidal box-girders. Two plexiglass models of box-girders are analyzed by the present method. The results are in good agreement with the experimental results, validating the proposed method and formulas. If the generalized warping displacement in restraint torsion is substituted by the first order of twist angle, the derived general differential equations will degenerate into the existing equations ignoring the secondary shear flow.