Abstract: The 15^# coal seam horizontal well in Jincheng Pangzhuang Block is difficult to form holes and shaft walls are liable to collapse, in order to solve these difficulties, research methods such as downhole observation of CBM well, optimization experiment of drilling fluid material and indoor evaluation experiment of drilling fluid system were adopted to optimize the weak gel drilling fluid system of horizontal CBM well, and the field application test was carried out in SHH02-4 horizontal well. The results show that weak gel drilling fluid system can ensure safe drilling, the residue is easy to degrade and has little pollution, the drilling cycle is 6% shorter than that of surrounding L - shaped wells, the horizontal drilling length is increased by 190 m, the well produced 1 320 m³ of CBM per day at the initial stage of drainage and production, it will be expected to achieve high production. The results show that weak gel drilling fluid system has good applicability in CBM horizontal wells.