Automatic reducing and broaching drilling tool

The utility model discloses an automatic variable-diameter borehole drilling tool used in chiseling in and drilling a plurality of gas drainage holes under a coal mine, which comprises an anterior drilling body, a cutter body, a posterior drilling body, a check ring and a union nut; the anterior drilling body is connected with the posterior drilling body by the check ring and the union nut, and the cutter body is fixed on the anterior drilling body by an axle pin, and a sliding wedge is arranged and fixed on the posterior drilling body by a fixed screw, and the head of the sliding wedge leans against the inner wall at the tail of the cutter body; the utility model has the advantages of flexible backing borehole variable-diameter, compact structure, convenient operation and so on.