Drill bit of rotary drilling rig for preventing drilling hole shrinkage in soft plastic stratum

The invention discloses a drill bit of a rotary drilling rig for preventing drilling hole shrinkage in a soft plastic stratum. The rotary drilling rig drill bit comprises dredging teeth, a sleeve, a disc, dredging knife edges and a butt joint body. The dredging teeth are uniformly distributed and connected to the lower end surface of the sleeve; symmetrical ventilation notches are formed in two sides of the periphery of the disc, the disc is connected with the upper end of the sleeve, straight-through pipelines are symmetrically arranged at the inner walls of two sides of the sleeve and communicate with the ventilation notches, the dredging knife edges are located on the lower portions of lower openings of the straight-through pipelines, the lower end of the butt-joint body is concentrically and fixedly connected with the top face of the disc, and pin holes are formed in the butt-joint body. The rotary drilling rig drill bit can prevent the rotary drilling rig from drilling in a soft plastic stratum to genera