The invention provides a device and method for transforming physical characteristics of a coal seam through downhole heat injection, and relates to coal mining technologies. According to the device and method for transforming the physical characteristics of the coal seam through downhole heat injection, the technical problem that rock burst or outburst of coal and gas easily occurs during coal roadway tunneling is solved. According to the device and method for transforming the physical characteristics of the coal seam through downhole heat injection, a coal seam heating system is composed of acoal mine neutral water purifier-softener, a high pressure water pump, an electric heater, a heat injection pipeline and a heat injection drilling hole; water is injected into the electric heater through the high pressure water pump after being processed through the purifying-softener, the water is heated by the electric heater in the high-pressure state, and the generated superheated water is injected into the coal seam along a hole sealing pipe of the heat injection pipeline and the heat injection drilling hole; after water enters the coal seam, part of water is converted into water vapor along with reduction of pressure, and the superheated water and the water vapor is subjected to seepage along cracks of the coal seam and heat the coal seam; and the water after heat is released flowsinto a pumping and mining drilling hole and is discharged from the coal seam. After the coal seam is heated for a period of time, the physical characteristics such as adsorption, the strength and thespontaneous combustion point of coal can be changed in a short time, and rock burst or outburst of coal and gas during coal roadway tunneling is effectively prevented.