The invention belongs to the technical field of superspeed impact loading tests and particularly relates to a piston retreater of a light-gas cannon. According to the technical scheme of the piston retreater of the light-gas cannon, an oil hole is formed in the sidewall of the piston retreater; the bottom surface of the piston retreater is provided with an annular groove used for mounting a sealing ring and an oil channel communicating with the oil hole. After completion of emission of light gas, a first-stage pump pipe and a second-stage pump pipe of the light-gas cannon are moved away. The piston retreater is placed at the head of a high-pressure cone section. The piston retreater and the high-pressure cone section are pushed through hydraulic pressure to realize sealing. The oil hole of the piston retreater is connected to a high-pressure oil pump. The high-pressure oil pump pumps oil into the high-pressure cone section through an oil inlet. A piston is retreated out of the high-pressure cone section through oil pressure. The piston retreater is simple in structure, high in reliability and capable of effectively solving the problem that a piston cannot be retreated out of a high-pressure cone section after emission of light gas.