Combustible ice, also called as Natural Gas Hydrate (NGH), is formed at the high pressure and low temperature, and much of the combustible ice is reserved in the seabed and tundra layer and is the third energy source human found. The invention provides a method and a device for exploiting combustible ice in the seabed by seabed firing. The method includes injecting combustion-supporting gas into a natural gas layer of the seabed combustible ice, strictly controlling flow velocity and flow of the combustion-supporting gas, firing through an electromagnetic ignitor, providing head for decomposition of NGH by burning a small part of natural gas, and accelerating decomposition of the combustible ice so as to improve gas production efficiency. Due to burning, the temperature of the natural gas is slightly higher than the balance temperature of the combustible ice, high pressure of gas reservoir of the combustible ice can be kept, and no hydrate of the integral seabed gas collection area is generated.