Abstract: A lifting surface analysis of vortex lattice method is completed. The coupling between the slipstream and the flow over the aircraft is obtained by using the Von Neumann condition at each control point of the aircraft and propeller panels. The slipstream contraction which definitely exists behind the propeller has been accounted. Practical computations for Y\|7 aircraft with double propellers show that the influences of the slipstream on the lift characteristics with deflected flaps and on the pitching moment coefficient of the complete aircraft are quite obvious. The computation results are in good agreement with experiment data. This method can also be applied to multi and single propeller aircraft.