Auxiliary Power Unit (APU) is an important subsystem of modern large and medium-sized passenger aircraft. This paper first used CATIA to construct a model of an aircraft and its APU inlet system, and then used Star CCM+ to simulate the APU inlet flow field. It was found that the ground wind direction has a small impact on the APU inlet total pressure recovery coefficient, but a significant impact on the inlet distortion. The crosswind not only significantly increases the APU inlet distortion, but also changes the direction of the APU inlet distortion. The position of the APU inlet also has an impact on the APU inlet distortion. Under 90° crosswind (the crosswind comes from the aircraft side where the APU inlet is located), the APU inlet distortion is maximum.