Abstract: Due to the development of wide-bandgap power electronic devices and motor technology, the specific power density and capacity of electric aircraft motors have been greatly improved, which can better satisfy the needs of more electric aircraft propulsion. However, extreme environmental conditions (low pressure, high temperature) and high-frequency square wave electrical stress with high dU/dt (fast rising/falling time) are more easily to cause partial discharge, which may accelerate the aging of insulating materials and lead to failure. PD detection is an important approach to evaluate the insulation reliability, whereas, unlike the PD detection under sinusoidal voltage, the square wave voltage source with fast rise time can cause strong electromagnetic interference, which greatly interferes with PD detection. In order to solve this problem, this paper introduces a combined detection method for partial discharge of more electric aircraft propulsion motor insulation based on UHF down-mixing method and silicon photomultiplier photoelectric method, which has strong anti-electromagnetic interference ability. To test the actual operating conditions of the aircraft, the proposed method is used to measure the partial discharge of the motor insulation sample when a square wave voltage is applied under different air pressures (down to 20 kPa). The interfacial charge relaxation process of solid-gas composite insulation of motor insulation is analyzed, and the antagonistic effect of interfacial relaxation charge and local residual charge on transient electric field under repeated square wave voltage is proposed. The improved Paschen formula of partial discharge initial voltage under fast square wave with different air pressure is obtained, and the influence law of low air pressure on local discharge characteristics under fast square wave is analyzed, which provides reference for insulation performance evaluation and optimization design of propulsion motor for more electric aircraft.