Abstract: In order to reduce the adverse effects of free water in the drilling fluid on the well wall stability and reservoir protection of CBM horizontal wells, the bound water drilling fluid, its formula was clear water +0.1% soda +0.5% to 0.8% bound water gelling agent + 0.5% to 1.0% water-based lubricant, was developed on the basis of evaluating the ability of the drilling fluid treatment agent to bind the free water. The performance evaluation results of CBM-bound water drilling fluid show that the free water content is low, the plugging ability is strong, the loss of the invasion coal rock is small, and the effect on the coal rock strength is little. The ability of stabilizing well wall and protecting coal reservoir of the bound water drilling fluid are better than xanthan drilling fluid which is used at present. The field application results show that the bound water drilling fluid can solve the contradiction between reservoir protection and wellbore stability of CBM horizontal well at present.