The invention relates to a marine natural gas hydrate depressurizing exploiting device and an exploiting method thereof. The marine natural gas hydrate depressurizing exploiting device includes a natural gas hydrate exploiting well, gas-liquid treatment equipment and data acquisition and monitoring equipment. The data acquisition and monitoring equipment is used for monitoring and controlling theliquid level, temperature and pressure conditions and working conditions in the natural gas hydrate exploiting well. The natural gas hydrate exploiting well is internally provided with a gas conveyingpipeline, a liquid conveying pipeline, a tail pipe and a tank type container. The upper end of the gas conveying pipeline is connected with the gas-liquid treatment equipment. The upper end of the liquid conveying pipeline is coaxially arranged in the gas conveying pipeline in an inserted mode, extends out of the side wall of the lower end of the gas conveying pipeline and then is connected withthe gas-liquid treatment equipment. The tank type container is arranged at the lower end of the liquid conveying pipeline and is internally provided with drainage depressurizing equipment and heatingequipment. The drainage depressurizing equipment is connected with the liquid conveying pipeline and the heating equipment. The bottom of the tank type container is connected with the tail pipe. The marine natural gas hydrate depressurizing exploiting device can be widely applied to exploitation of marine natural gas hydrate.