Abstract: Accurate calculation of deflection of composite beams with elastic shear connections is difficult because of difficultly getting an analytical solution under an arbitrary load, and analytical solutions in common load cases are quite complicated. For these reasons, a simplified calculation method called method of reduction of stiffness was provided in Chinese Code for Design of Steel Structures. However, the results obtained by the method are not accurate enough and its application is limited under some circumstances. In order to solve the above problem, a new method called effective stiffness method was proposed. Calculations with this method are simple and accurate, and the proposed method has a clear mechanic background and can give a composite factor showing the actual effect of shear connections. It can be used to composite beams with an arbitrary stiffness of shear connections, i.e., from near zero to infinity. The calculation results show that the relative error between its solutions and analytical solutions is less than 1.0%.