Abstract: During the flight, the mass flow through the engine is controlled according to flight mission requirements. The pressure distribution on the nacelle lip will change significantly, as well as the total force of the outflow acting on the inner stream tube ahead of the intake, consequently, the spillage drag will be changed. The spillage drag for a civil aircraft at different mass flow is obtained through the combined method of CFD powered simulation and thrust-drag bookkeeping. Decisive issues to determine the spillage drag, such as the mass flow rate, flight Mach number, altitude and angle of attack, are analyzed. The prediction method and the investigation findings of the spillage drag can be referenced in aircraft/turbo-fan engine integration design, as well as the high-bypass-ratio nacelle design and related aerodynamic predictions.