Multipacting in the reduced-height waveguide coupler used in CESR III superconducting RF system is studied through numerical and simplified analytical calculations. An improved model for secondary electron emission is developed for numerical calculations. We present in this paper a new method to counter against multipacting by opening slots on broad walls along the center plane of a rectangular waveguide. The simplified analytical approach provides a fast survey of multipacting characteristics for various VSWR's in the waveguide.