Abstract: Exact analysis of membrane stress distribution of thin-walled beam bridge under both vertical and horizontal loading is significant for the calculation of shear log and will be beneficial for finally determining the efficient width of bridge wing for practical design purpose. By generalizing force method into a 2-D structural problem, a typical flat-arch-shaped beam bridge is divided into a slab of plane stress and 2 beams of rectangular cross section. Consequently, an unknown distributive shear is assumed on longitudinal intersection of both beam and slab. By adapting elasticity theory, a complete system of Airy stress functions, which satisfies both biharmonic controlling equation of 2-D plane stress problem and its boundary conditions, is employed for the analysis of stresses and displacements of slab. The beam ribs are also analytically solved for their deflection and rotation displacements by Timoshenko's theory. The deformation compatibility, that the longitudinal displacements of both beams and slab are equivalent to each other at the longitudinal intersection, is used to finally determine the unknown shear. The efficiency of analytical solution procedure is verified and demonstrated by examples and an excellent agreement is reached with the results of FEM and model experiment of Yangtse Highway Bridge at Tongling.