The cooling air is introduced to air conditioning cooling system of aircraft through ram air inlet duct. In order to provide more ram air mass flow for the heat dissipation of aircraft electronic equipment, an ejector is considered to be installed in the ram air inlet duct. In view of the two flight heights of 5 km and 10 km, a numerical method is used to study the flow field of ram air inlet duct with and without ejector with flight Mach number of 0.2—1.2. The results show that, at the same height, the mass flow increase ratio in the ram-air inlet duct with ejector decreases with the increase of the flight Mach number . At low height and low speed (the flight height is 5 km and the flight Mach number is 0.2), the ejector has a better effect on increasing the mass flow rate, and the increase of the mass flow rate in the ram air inlet duct is about 96.87%.Thus, the research in this paper can provide theoretical reference for the improvement of air conditioning cooling device for aircraft.