Shale oil in Qikou Sag is an important replacement resource for the development of Dagang oilfield on land. When drilling the well, QY6-36-1, a shale oil horizontal well in Qikou with Water-based drilling fluid, serious wall instability occurred. After twice sidetracking, Oil-based system was replaced to complete drilling. Aiming at this problem, the hole stabilization mechanism of shale oil formation was analyzed, and the coupling mechanism of “stress—chemical—thermos—stratification” in shale oil formation was clarified. The formulation of oil-based drilling fluid was studied to suitable for shale oil formation in Qikou sag, key performance parameters were optimized, and technical measures for sealing and anti-collapsing were provided. Finally, a set of oil-based drilling fluid system was formed. It has good rheology and excellent sealing capacity. The Oil-based drilling fluid was applied to 3 pilot-testing Wells, and had outstanding effect on wall stability. There was no complex downhole accident. The average mechanical drilling rate is 15.53 m/h. It is 70% higher than the drilling rate of the 9 completed Wells, which drilled with Water-based drilling fluids. It provides technical support for the efficient exploration and development for shale oil in Dagang oilfield.