Abstract: Adifferential equation for calculatingthe normal stress atthe transverse section of the flange of a steel-concrete composite box girder is derived based on the compatibility of strain and the equilibrium of forces on an micro element of the flange section. Separation of variables is employed to solve the differential equation under the boundary condition of two ends of the girder being simply supported, and the solution of stress is obtained in the form of Fourier series. The shear lag coefficient is then obtained according to its definition. An application example proves the quick convergence and high accuracy of the calculating method presented in this paper, which also shows that satisfactory results are achieved through simple calculations.