Abstract: In order to strengthen the drainage effect of CBM horizontally-butted wells in later stage in Binchang Mining Area, under-balanced drilling technology is adopted for the construction of horizontally-butted wells. In this process, through establishing the mathematical model, the key technical parameters of annular pressure loss, the range of bottom hole pressure and flow control of gas injection in vertical well are calculated. Moreover, this technology is adapted to drilling technology in Binchang Mining Area butted well group. Mathematical calculation and field application results show that it is suitable for the Binchang Mining Area to adapt under-balanced horizontally-butted wells. When the horizontal section length is less than 600 m, the injection volume of 0.45 m³/min of liquid and 35.5 m³/min of gas can be set to achieve the under-balanced state of bottom of well; when the horizontal section length is more than 600 m, the injection volume of 0.6 m³/min of liquid and 35.5 m³/min of gas can be set to achieve that part of wells are controlled in the slightly over-balanced state so as to realize the large footage in coal seam, to effectively protect the reservoir and greatly increase the production in later stage.