Gas hydrate cool storage systems have been developed since the end of 1980s. Compared with the prevalent ice-storage systems, gas hydrate cool storage systems are characterized by higher phase-change temperature and very large latent heat released during phase-change process. Higher phase-change temperature can avoid the decease of the COP of water-chiller unit and make the air-conditioning system more effective. This paper analyzed the complicated process of heat and mass transfer inside an indirect-contact gas hydrate cool-storage tank. It's indicated that condensation, evaporation and hydration are a combined and mutually-influenced process. A qualitative model is disgust to explain the behavior of hydrate cool storage.