Abstract: The key problems on multi-element airfoils optimized for maximum lift arethe false calculation of the target function and how to select the suitable indirect target function to greatly decrease the CPU time. Based on a great deal of calculation, the valid method is obtained as follows. The condition that the confluent boundary layer can not form too early is used to restrict the gap between airfoil elements not too small, thus solving the problem of false caculation. For the optimization of a leading-edge slot, the separating position of the main element is used as indirect target function for optimizing caculation; and for that of a trailing-edge flap, the lift of the airfoil or separating position at the key airfoil element is used. Then a few further caculation with a target of maximum lift coefficient are carried out. The reliable results can be obtained repidly.