The invention discloses an inner bore supersonic speed flame gun for a liquid fuel. The flame gun comprises a gun housing, a gun barrel, an oxygen conveying pipeline, a fuel conveying pipeline, an ignition line pipeline, a fuel spray nozzle, a stabilizer, a combustion chamber, a high-temperature high-pressure gas gathering section, an elbow connector, a power feeding needle, a gas compressing throat, a flame diffusion section, a nozzle, a bottom water outlet of the combustion chamber, a sparking plug and a cooling water inlet pipeline; the combustion chamber is located in the gun barrel, the fuel spray nozzle is arranged in a position where the fuel conveying pipeline and the stabilizer are connected, the sparking plug is arranged in a position where the ignition line pipeline is connected to the stabilizer, the stabilizer is arranged at one end of the combustion chamber while the elbow connector is arranged at the other end, the high-temperature high-pressure gas gathering section, the gas compression throat and the flame diffusion section are successively arranged in the through pipeline after the combustion chamber is connected to the elbow connector, and the nozzle is arranged at the tail end of the flame diffusion section; the power feeding port is located in the middle position of the elbow connector. The inner bore supersonic speed flame gun disclosed by the invention can effectively improve the flame speed and the bonding strength of the coating and reduce the porosity.