For the APU/main engine refuel tube within fuselage contour, some critical leakage malfunctions were studied by numerical simulation method based on Fluent. After analyzing the flow feature of APU/main engine refuel tube leakage, which caused by the missing of one seal within the inner flexible coupling, the relationship between leakage flux and refuel pressure can be concluded that the leakage flux increases with refuel pressure increasing. Besides refuel pressure, the leakage flux is influenced obviously by the tube size and the tube manufacture precision, while the simulation result also shows that the simultaneous missing of a clamshell body affects the leakage flux little.