Abstract: This paper presents a simple way of calculation shear lag in prestressed concrete truss bridge deck which is subjected to horzontal axial forces, vertical force, and moments from diagonal web members and external loads. Governing differential equation for shear lag is obtained by variational approach under the combined action of horizontal force and vertical force, and the stress of considerating shear lag is obtained on linear condition by means of superposition principle. A calculation example is illustrated and compared with test result.