The invention belongs to the technical field of superhigh-speed collision loading tests and particularly relates to a three-stage light gas gun. The three-stage light gas gun based on hydrogen and oxygen mixed detonation driving adopts the technical scheme that the ratio of the inner diameter of a first-stage pump pipe to the inner diameter of a second-stage pump pipe is 2-4, and the ratio of the inner diameter of the second-stage pump pipe to the inner diameter of a launching pipe is 3-5; the draw ratio of the first-stage pump pipe is 3-5, the draw ratio of the second-stage pump pipe is 4-7, and the draw ratio of the launching pipe is 1-2; the conical angle of a first-stage high-pressure conical section is 5-25 degrees, and the conical angle of a second-stage high-pressure conical section is 5-25 degrees; and a combustion chamber is filled with mixed gas of hydrogen, oxygen and nitrogen, the first-stage pump pipe and the second-stage pump pipe are separately filled with hydrogen, and vacuumizing treatment is carried out on the launching pipe. The three-stage light gas gun adopting the hydrogen and oxygen mixed gas as a driving source is stronger in loading capability, is low in pollution and stably launches 1g-mass shots at a speed higher than 10km/s.