Abstract: In horizontal drilling in the Fauqi oilfield, the angle build and hold sections are generally at depths between 3 350-4 480 m, the Cretaceous system, which mainly composed of limestone, mudstone and shale. Long time of hydrocarbon development has resulted in low formation pressure. During drilling, borehole collapse, differential pressure pipe sticking and lost circulation have been encountered frequently. Based on the formation characteristics, a BZ-KSM drilling fluid was used in drilling the well FQCS^**. To improve the rheology of the drilling fluid and to prevent the formation of cuttings beds, an additive BZ-YRH was introduced into BZ-KSM. BZ-YFT, a nano plugging agent and a silicate were used to improve the inhibitive capacity of the drilling fluid. To avoid lost circulation, mud density was maintained between 1.22 g/cm³ and 1.28 g/cm³ in the 5th interval. Prior to POOH to change to directional drilling tool, the mud density was increased to 1.35 g/cm³. Field application has shown that this drilling fluid had good inhibitive and plugging capacity, as well as superior filtration control performance and rheology. Downhole problems such as shale dispersion, borehole instability and difficulties in cuttings carrying have been greatly mitigated, ensuring the success of the drilling operation.