Aircraft range and duration can be effectively increased by mounting auxiliary fuel tank, and the effect of external auxiliary fuel tank on aerodynamic characteristics of the aircraft and flight dynamics must be taken into account to ensure flight safety during an aircraft design or modification. The lift-drag characteristic of the aircraft with wing-mounted auxiliary fuel tank can roughly be obtained using traditional engineering estimation methodology, whereas the aircraft aerodynamic characteristics caused by the interference of wing-mounted auxiliary fuel tank and aircraft can’t be predicted accurately. However, an effective method for investigating the effects of wing-mounted auxiliary fuel tank and its pylon on aerodynamic characteristics of the aircraft can be provided by CFD technology. In this paper, the CFD assessment method of the effects of wing-mounted auxiliary fuel tank and its pylon on aerodynamic characteristics of the aircraft is investigated, and the principles to be followed in the assessment methodology is proposed firstly. Secondly, the influence law and quantitative conclusion of the effects of wing-mounted auxiliary fuel tank and its pylon on aerodynamic characteristics of an aircraft is obtained, which provide the important evidence for the modification design of an aircraft.