The invention belongs to the field of aerodynamic layout design of civil airplanes, and relates to a design method of a slotted flap of an airplane. The design method comprises the steps that a stalling point on the upper surface of a wing in take-off and landing states is calculated, and a point A behind the stalling point is taken and used as a dividing point between the flap and a main wing; a point B is taken, a curve AB serving as the leading edge of the flap is designed, a rotating shaft center point O of the whole flap is designed collaboratively, and therefore a contraction air channel can be formed between the rear edge of the main wing and the leading edge of the flap when the flap is put down, and airflow on the lower surface flows through a gap and then blows away, wherein the point B meets the conditions that the distance between the point A on the lower surface of the wing and the point B is 0.65 time the chord length, and the point B is located in front of the point A; the design is optimized by means of calculation according to CFD; confirmation is conducted by designing and processing a wind tunnel test model and carrying out wind tunnel test research. By means of the designed flap with the rear slotted edge, the situations that the flap can deflect by a certain angle when the airplane is in a take-off stage and a landing stage, and lifting power as high as possible is provided at a low speed can be guaranteed.