We have proposed a new open-cycle desiccant air-conditioning system using a thermo-sensitive water vapor adsorbent that has a phase transition temperature below 333 K, which is expected to result in energy saving. In this system, it is necessary to suppress the rise in the temperature of the adsorbent to be below the phase transition temperature during adsorption. In this study, the performance of a plate heat exchanger type adsorption reactor that has a function of cooling the adsorbent bed was evaluated using a simulation model as well as some other experiments. This model simulated well the characteristics of water vapor transfer and heat transfer in the adsorbent bed. In addition, the simulation results showed that heightening the effective thermal conductivity of the adsorbent bed was more effective than decreasing the coolant temperature in order to obtain the improved effect of the stable cooling capability of the adsorbent bed over time. This simulation method is very useful for the configuration design of a plate heat exchanger type adsorbent reactor.