Air mixing system which is composed of air pressure control system, hot pipe system and air mixer, is the facility for mixing hot air($1000^{circ}C$, 10kg/s) from storage air heater (SAH) and decompressed air($20^{circ}C$, 15kg/s) from high pressure air supply system. Air pressure control system reduce the pressure of the air, from 32MPa to 3.5 MPa and supply the decompressed air to air mixer. The hot pipe system supply hot air from SAH to air mixer which mix hot with the decompressed air from air pressure control system. Fully mixed air flow rate is 25kg/s and mixed temperature is up to $400^{circ}C$. So, we can expand the operating envelop of the supersonic ground test facility to low Mach number and low altitude region.