The invention relates to an in-situ submarine natural gas hydrate exploiting device and a method thereof, and belongs to the technical field of exploitation of natural gas hydrates. The exploiting device comprises a solar and wind power generating system, a rotary telescopic water gun, an insulating pipeline, a conveying pipeline, a sleeve pipe, an ocean platform, a sea water pump, an electric heater, a CO2 liquid storing tank, a decomposition accelerator storing tank, a high-pressure pump, a pressure gauge, a filtering device, a water storing tank, a gas-liquid separator and a gas recovering device. A thermal activation method and a chemical reagent catalyzing method are utilized to trigger a decomposition reaction of the natural gas hydrate, and then the high-pressure CO2 jetting technology is utilized to enable the natural gas hydrate to be cut while a substitution reaction occurs. According to the invention, the natural gas hydrate is exploited by fully utilizing rich solar power and wind power in ocean areas, so that the effects of cleaning and environmental protection are realized; the thermal activation method and the chemical reagent catalyzing method are matched with the CO2 substitution method, and therefore the exploitation rate is improved, the dosage of a chemical reagent is reduced, the stability of submarine geology is protected effectively, and meanwhile the greenhouse gas is treated to a certain extent.