The heat and water vapor transfer characteristics of a rotary regenerator are important factors for improving the cooling and dehumidifying performance of a desiccant air-conditioning system. The objective of this study is to optimize the design parameters of a plastic rotary regenerator. A numerical model was developed and validated to predict the performance of a plastic rotary regenerator considering leakage and adsorption. The effects of the rotating speed, split, aspect ratio, and purge section angle on the heat and water vapor transfer effectiveness were analyzed using the numerical model. The optimum rotating speed, split, aspect ratio, and purge section angle in the plastic rotary regenerator were determined to be 8rpm, 0.55, 0.35, and 4°, respectively. The maximized heat transfer effectiveness and the minimized water vapor transfer effectiveness with use of the optimal design were 0.85 and 0.042, respectively, which yielded a 39.6% improvement in the water vapor transfer effectiveness compared to that with use of the basic design.