The invention relates to an active rotary telescopic compound bit applicable to a deep difficult-to-drill hard formation. The bit mainly comprises a bit body, a power end casing, a rack piston cylinder, a narrow-mouth piston, a wide-mouth piston, an arc-shaped rack, a gear, a drilling fluid through hole, a telescopic column, a three-hole flat plate, legs, cones, cone teeth, runner ports and drainage ports, wherein the narrow-mouth piston is connected with the wide-mouth piston by the aid of the arc-shaped rack; the gear embedded inside the rack piston cylinder is meshed with the arc-shaped rack, and the drilling fluid through hole is formed in the center of the gear; the lower end of the gear is fixedly connected with the telescopic column, the tail end of the telescopic column is fixedly connected with the three-hole flat plate, PDC (polycrystalline diamond compact) blades and the cones are arranged at the lower end of the bit body simultaneously, and the PDC blades are evenly arranged in the circumferential direction of the outer wall of the bit body; the telescopic column drives the cones on the three-hole flat plate to axially penetrate through the central groove, and axial movement of the cones is realized. According to the compound bit, the drilling fluid flow is controlled actively, so that the rock breaking efficiency is substantially improved, the drilling cycle is shortened, and the drilling cost is reduced.