Abstract: According to the problems of shallow gas drainage drilling hole, high accident rate and other issues in soft coal seam, we designed a retrievable bottom hole assembly, the drilling of the coal seam gas drainage construction is divided into two levels to effectively extend the drilling depth. Based on the turbulence model of bottom hole assembly and FLUENT software, we analyze the work characteristics of gas-phase velocity and gas-phase pressure inside the drill field, and obtain that air flow ratio of two watershed channel of bottom hole assembly is 8: 2, which meets demand of the hole bottom drilling.