The invention relates to a method and a device for exploiting deep sea combustible ice. The method comprises the following steps of: transporting the combustible ice to a supercritical water oxidation reactor in seabed, wherein the supercritical water oxidation reactor is filled with water medium; sending preheating oxygen to the supercritical water oxidation reactor from an external pump, wherein the pressure of the oxygen is equal to or a little more than the pressure in the supercritical water oxidation reactor; igniting in the supercritical water oxidation reactor; and performing an oxidation reaction on the combustible ice, wherein the heat generated through reaction is sent to seal level through heating water vapor by a deep sea steam heater so as to drive a steam turbine to generate electricity, the concretes and non-condensing materials from the deep sea steam heater are separated, and a part of the separated water returns to the supercritical water oxidation reactor to be used as evaporable water. By the invention, the combustible ice can be oxidized under a deep sea supercritical water state; and oxidation exothermic is used for generating electricity, so that the combustible ice can be prevented from volatilizing in the process of transporting to the sea level, and environmental risk is eliminated.