Abstract: For the need of pitch balancing control, the body flap is usually arranged on the base of the plane symmetry aircraft with the longest moment arm. Due to the close distance between the body flap and the engine nozzle, the body flap is easily disturbed by the engine jet. Although the interference force is far less than the thrust of the engine, its direction is perpendicular to the body flap, and the moment arm is significantly longer. As a result, the pitching moment generated by the jet interference is not small as compared with the pitching moment generated by the thrust of the engine, and even adversely affects the control of the aircraft. By solving the multi-component N-S equation numerically, the mechanism of jet interference flow field between the engine and body flap was analyzed, and the reason why the jet interference characteristics are completely opposite at low altitude and high altitude was found.