Abstract: Aiming at the influence of different liquid oxygen filling volumes on the transient phase change expansion effect of liquid oxygen, the propagation laws of gas front and mixed gas generated by liquid oxygen phase change expansion in free field, as well as the variation characteristics of gas pressure, are investigated by using a high-speed schlieren system and an overpressure measurement system. Three charge cartridges with a diameter of 20 mm and lengths of 50 mm, 70 mm and 100 mm respectively are designed, the liquid oxygen filling volume is controlled by the cartridge size, and two pressure sensors arranged at different positions are adopted to realize comprehensive measurement of gas pressure variation. Schlieren images at typical moments during gas propagation are selected to analyze the wave variation of gas front and mixed gas, the gas propagation process, velocity variation and pressure variation during liquid oxygen phase change expansion under three liquid oxygen filling volumes are compared and analyzed, and the propagation laws, evolution processes and pressure distribution characteristics of mixed gas in free field are clarified by combining theoretical analysis with experimental results. Results show that, after the liquid oxygen absorbent is ignited by the ignition head, a large amount of heat is released by the absorbent to induce rapid phase change expansion of liquid oxygen. The gas is continuously accumulated, heated and pressurized inside the charge cartridge until the burst pressure limit of the cartridge is reached. After that, the gas front and mixed gas propagate outward in a spherical pattern with the cartridge as the center, presenting a mode of initial accompanying movement and subsequent separation. With the increase of liquid oxygen filling volume, the gasification and energy accumulation efficiency of liquid oxygen is improved, the separation time is advanced with the increase of liquid oxygen filling volume, and the pressure accumulation time is reduced from 2.91 ms to 1.20 ms, with a reduction rate of 58.76%. The propagation velocity of the gas front is increased, with the peak velocity rising from 1025.64 m/s to 1487.18 m/s, an increase rate of 45%. The peak overpressure at measuring point 1 is increased from 35.65 kPa to 40.93 kPa, with an increase rate of 14.57%. The positive pressure duration and peak impulse present a nonlinear growth trend, which reach the maximum at the cartridge length of 70 mm and decrease at the cartridge length of 100 mm. Excessively long charge cartridges may cause dispersion of phase change energy, or induce turbulence and reflected wave interference due to unbalanced length-to-diameter ratio. Although the peak overpressure can be increased under this condition, the continuous energy action efficiency is reduced.