The invention relates to a heating device for a water jacket furnace, a water jacket furnace and a heating method of the water jacket furnace. The heating device of the water jacket furnace comprises a main fire nozzle and an auxiliary fire nozzle, wherein the main fire nozzle is located in a fire cylinder of the water jacket furnace and is communicated with a natural gas source; the auxiliary fire nozzle is located in the fire cylinder of the water jacket furnace, is communicated with the natural gas source and an air source and can form mixed gas; the mixed gas sprayed by the auxiliary fire nozzle can be ignited by the flame generated by the main fire nozzle. Through the heating device for the water jacket furnace, the water jacket furnace and the heating method of the water jacket furnace, the integrated heating of the whole fire cylinder can be achieved; the heating area can be maximized; the heating efficiency is high; the volume ratio of the natural gas to the air can be digitally regulated; the most complete combustion can be achieved; the consumption amount of the natural gas can be reduced; and the purpose of saving energy and reducing emission can be achieved.