One characteristic of B737 airplane is that the slotted extension state of the leading edge slats can be applied in the takeoff, while other types of airplanes do not apply this state in the takeoff. In this paper, the relationships between the flap deflection and the following four parameters of the takeoff were analyzed: the maximum takeoff weight limited by field length, the maximum takeoff weight limited by climb gradient, the maximum takeoff weight limited by obstacles and the takeoff decision speed. The results show that these four parameters of takeoff are abnormal when the flap deflection is 10. The reason is that the extension state of the leading edge slats is changed. During the increase of the flap deflection from 5 to 10, the extension state of the leading edge slats is changed from the sealed extension state to the slotted extension state, and such a change of state causes a decrease of the lift coefficient.