More and more multi-electric aircraft use power electronic equipment, and this kind of equipment is equivalent to constant power load, with negative impedance, which is easy to affect the stability of the aircraft power supply system, especially the high-voltage DC power supply system of multi-electric aircraft. In this paper, the average model equivalence of the multi-electric aircraft HVDC power supply system with constant power is carried out, and the stable load limit of the multi-electric aircraft HVDC power supply system is determined by using the Middlebrook stability criterion combined with the bode diagram. Then, the steady-state power balance point of the power supply system is verified by taking a certain type of aircraft power supply system as an example.