Abstract: In order to improve gas drainage effect of crossing borehole, cavity drilling technology is often used to increase part borehole diameter of coal seam section, which avoids invalid footage of hole section in rock layer. However, due to limitation of structure of cavity drill bit, it is generally necessary to to lift the drill after the pilot drilling is completed, then the cavity drilling bit is put into the hole to carry out cavity drilling, which need more procedures, long auxiliary operation time. In order to solve the above problems, a mechanical cavity drill bit while drilling was designed with rack and pinion structure. The wing is retracted when the cavity drill bit is drilling in the rock layer. After entering coal seam, axial movement of the piston with rack is driven by change of hydraulic parameters, which drives the pinion blade to rotate and open to ream drilling. After caving drilling is finished, the wing is retracted, the drill bit is pull out from the drilling hole, and the hole is sealed and the gas is pumped. The field test results show that the drill bit can construct pilot hole when rock is closed, and can open smoothly and carry out hole cutting drilling after entering coal seam and the pump volume is increased, which increases the diameter of borehole in coal seam, reduces auxiliary operation time of crossing borehole,improves construction efficiency of crossing borehole.