The terminal diameter and depth are 1200mm and 423.66m respectively in 4# gas drainage borehole of Tashan coal mine of Datong Coalmine Group Company with displacement less than 2m. The main difficult points in this project are large borehole diameter, deep drilling and high precisiom. Multi-process of pneumatic DTH hammer, double-wall drill pipe air lift reverse circulation and combined cone multilevel reaming were adopted to improve drilling efficiency, ensure wellbore quality, and achieve good economic and social benefits.