According to the borehole instability of coal bed and carbonaceous mud strata and support pressure of directional drilling in the course of horizontal drilling in Jingnan region of Erdos, the study was carried out on anti-caving and formation blocking as well as lubricating and resistance-reducing technologies of drilling fluid. A drilling fluid formula with good anti-caving and lubrication ability was developed, which brought shale rolling recovery ≥98% and extreme pressure lubrication coefficient up to 0.05. In the application tests of 2 wells in the field, the average well diameter expansion rate in application well sections were 1.71% and 4.06%, respectively and the drilling tool friction resistance was 40～80kN. Drilling and well completion was conducted successfully.