Abstract: Some methods to calculate the deflection of composite beams with partial shear interaction were summarized, and the characteristics of these methods were discussed through the mid-span deflection of a simply supported beam acted by uniformly distributed load. The drawbacks of the reduced stiffness method (RSM) used in the current code for design steel structures were analyzed theoretically and a modified reduced-stiffness method (MRSM) for calculating the deflection of composite beams was proposed. Theoretical analysis and numerical examples show that the MRSM can overcome the drawbacks of the RSM, avoiding the conflict of bending stiffness of composite beams decreasing with the stiffness of the shear connectors. The MRSM can also take into account the influence of boundary conditions on the reduced stiffness and has a good agreement with the exact solution.