The APU compartment ejector drainage is a kind of complex mixture flow with gas and fluid. A 1-Destimation method for the outlet velocity of APU compartment ejector drainage system is obtained based on proper assumption. The outlet velocities of APU compartment ejector drainage system on ground and typical status in flight are calculated according to the estimation method. The calculation results can be the input for CFD simulation of APU compartment drainage path at aircraft level. The calculation results show that the ejector effect in the APU compartment ejector drainage system can increase the fluid drainage velocity effectively, and the fluid drainage velocity is inversely proportional to the altitude.