The SichuanTibet Railway is located in the southeast of the QinghaiTibet Plateau. Due to the development of tectonic activities, the rock mass is loose and broken with well developed fractures and confined water; thus, complex situations such as hole collapse, falling stones, stuck drilling, and lost circulation often occur during the exploration of these formations. In view of the problems of hole wall instability and lost circulation during the drilling of the directional horizontal holes designed for the formation along the tunnel exit section, a suitable drilling fluid system for the hole was proposed through analyzing the formation characteristics and leakage in the hole. Then, orthogonal experiments were carried out to quickly obtain the optimal combination of the adding content of the sealing materials. The follow up field application results verified that the improved drilling fluid worked well. Therefore, the drilling fluid with the optimized formulation can greatly reduce the incidents in the holes, effectively promote the exploration progress, and obtain significant economic and social benefits.