In order to study the lift enhancement effect of the retractable belly flap on the flying wing configuration, a flying wing configuration model was taken as the initial shape of flying wing configuration aircraft, the distribution of crevices on the retractable belly flap are studied by the method of numerical simulation, and the variation of aerodynamic forces under different retracted states of the belly flap is obtained. Results show that, the retractable belly flap, when it is fully extended, can effectively lighten the air separation behind it and enhance the lift. When the crevice width is the same as the width of the hill shaped rudder, the effect of lift r enhancement is the best. The pitching moment shows a quasi-linear growth with the decrease of the height of the retractable belly flap during the belly flap retraction process. Moreover, the lift enhancement effect of the retractable belly flap is better than that of the seamless belly flap at the same height.