An internal arc plasma gun relates to plasma heating equipment, and consists of a front gun body, a back base and a first electrode, wherein an arc cavity and a cooling water jacket are arranged inside the front gun body; the head part of the first electrode stretches into the arc cavity; the arc cavity and the head part of the first electrode stretching into the arc cavity form an internal arc generator; a metal component at an inlet forms a second electrode; and when the internal arc plasma gun works, an arc is generated inside the arc cavity, sprayed out from a compressing nozzle, and intensely heated after being compressed so as to form a plasma torch. By decomposing water molecules inside the arc cavity, energy of the high-temperature plasma arc is concentrated, the internal arc plasma gun has the characteristics of high heating efficiency and high decomposition rate of the water molecules, is applicable to a daily garbage gasifying furnace, a coal gasifying furnace, an industrial polymer waste gasifying device, a water-pyrolyzing hydrogen making device and a boiler, and compared with the conventional technology, can reduce the consumption of coal resources, improve the quality of synthesized gas and reduce emission of greenhouse gas.