Abstract: Based on the Timoshenko beam theory and the energy-variation principle，this article makes some corrections to the warping displacement function of thin-walled single-cellbox girder considering the warping displacement amplitude difference between different flanges，axial force equilibrium condition across section，and the shear deformation effects of webs on warping displacement function and the expression of total potential energy is built． Euler-Lagrange equation is used when the structure under the stable equilibrium state，and the differential equation of deflection in simply supported beam is deduced． Combined with the finite element method ( FEM) software ABAQUS，the beam deflection distribution along the length of the beam has been analyzed． The results show that the deflection distribution calculated by the theoretical analysis in this article is consistent with those obtained from FEM analysis． Meanwhile，a widely used deflection formula for engineering application is introduced for analyzing the deflection of simply supported beam． On account of neglecting the shear lag affect，which can generate the additional deflection，this widely used formula can have big errors by comparing the results with FEM results，up to 32. 06%，the error range can be 21. 39% ～32. 06%． However，the deflection distribution along the length of the beam in this article is in a good agreement with FEM results，which can better reflect the actual beam deflection under different loads． Consequently，the proposed method has been validated to be accurate and feasible． 

Key words: thin-walled box girder, shear lag effects, Timoshenko beam, energy-variation principle, deflection calculation