Abstract: In general, a transformed section method is adopted to calculate stresses of composite beams, without consideration of an interface slip. However, if a partial shear connection is used for composite beams or fatigue behaviour need to take into account, neglecting the effect of slip will lead to a great error. This paper deduces the analytic solutions of flexural stresses and interface shear forces in the elastic range of simply-supported composite beams under various load cases. On the base of this development, the variation rule of flexural stresses and interface shear forces under various load cases is obtained. According to practical requirement, the analytic solutions are simplified at the most unfavorable positions. Through a comparison, the simplified solutions show a good agreement with analytic solutions.