The invention discloses a fuel gas generating device which comprises a fuel gas fire grate, a fin heat exchange part and a smoke exhaust pipe. The fin heat exchange part is arranged above the fuel gas fire grate and comprises a water inlet pipe and a water outlet pipe. The smoke exhaust pipe corresponds to the fin heat exchange part. A waste heat water tank is arranged around the fin heat exchange part, wherein an interlayer structure of the waste heat water tank is formed by an inner container and a shell. A cold water tank is arranged around the smoke exhaust pipe which is communicated with the inner container of the waste heat water tank. The cold water tank is communicated with the waste heat water tank. A water inlet of the fin heat exchange part is communicated with the waste heat water tank. As the water tanks are arranged around the fin heat exchange part and the smoke exhaust pipe respectively, when the fuel gas generating device works, waste heat dissipated by the fin heat exchange part and the smoke exhaust pipe to the periphery can be absorbed by water in the water tanks to be turned into hot water, the hot water in the water tanks enters the fin heat exchange part to be heated, the amount of adopted fuel gas can be saved, the energy utilization rate is increased, and heat efficiency is improved.