Double-stage drill bit

The invention discloses a double-stage drill bit. The double-stage drill bit comprises a drill bit body, fixed blades, fixed cutting teeth, chip grooves, nozzles and the like and is characterized in that the core of the drill bit is inwardly concave to form a core cutting structure, and the core cutting structure and an outer cutting structure form the stepped double-stage drill bit. By the structure, the double-stage drill bit has the advantages that the problems that the drill bit is low in mechanical drilling speed and poor in stability when the drill bit is in a large well hole of a deep stratum are solved effectively, the outer cutting structure cuts annular crushed zones in the well hole first to form annular stress release faces in well-bottom rock, and drill bit rock breaking efficiency is increased favorably; protruded rock is formed at the core area of the drill bit, and drill bit stability is increased favorably; the well-bottom drilling parameters of large-size drill bits can be changed, double-stage cutting structure drilling pressure and torque proportion can be reasonably adjusted and allocated, and drilling efficiency is increased.