Abstract: In this paper，the power coupling efficiency of semiconductor laser passing through a collimating system is discussed. The relational expression for power coupling efficiency of semiconductor laser beam and k (ratio of the aperture radius to the equivalent radius of the laser beam) is given, and the relationship for the power coupling efficiency of semiconductor laser beam and NA (numerical aperture) of the collimating system is also offered according to the laser collimating system. The research will have a significant function for the design of semiconductor laser collimating system.