Abstract: Aiming at the reasonable structure design of large-diameter reverse circulation drill bit used in gas drilling, research on the relationship between undersurface figure of reverse circulation drill bit and forming effect of reverse circulation was carried out in this paper. Firstly, the flow characteristics of compressed air under reverse circulation drill bit were analyzed. Then the experimental tests were done with experimental apparatus for hole-bottom flow field simulation of special reverse circulation drill bit, three types of undersurface figure:convexity, concave and combination of convexity and concave were designed for reverse circulation drill bit and tested with regard to forming effect of reverse circulation, the results show that concave undersurface figure is beneficial to formation of reverse circulation. Finally, with CFD technology, the numerical simulation of bottom flow field formed by reverse circulation drill bit with different undersurface figure was done, which exposes the flow characteristics of compressed air under the drill bit and the mechanism of reverse circulation forming affected by undersurface figure. The research on the undersurface figure design is helpful to improve the performance of large-diameter reverse circulation drill bit.