Abstract: In order to address the problems of borehole caving, blockage and sticking while drilling the Paleogene shale formation in the Xijiang Oilfield of the South China Sea, technical research has been carried out on anti-sloughing drilling fluids. Through analyses of stratigraphic mineral composition, physicochemical properties and mechanical parameters, the mechanisms of wellbore instability in the Paleogene shale formation have been clarified, and have established the relationship chart between the drilling fluid density required for maintaining wellbore stability and rock cohesion, so as to determine the minimum rock cohesion index required to sustain wellbore stability. In order to improve the strength of surrounding shale immersed in drilling fluid, a new anti-sloughing drilling fluid formula was obtained through selection of proper inhibitor and plugging agent as well as their optimal dosages. The research showed that the intrusion of drilling fluid filtrate would lead to the reduction of shale strength, which is the main reason for the wellbore instability of Paleogene shale formation in this oilfield; After immersing for 10 days in the new anti-sloughing drilling fluid added by 2.0% polyammonium salt, 0.5% nano-silica and 3.0% calcium carbonate into KCl-polymer drilling fluid, the rock sample still has the cohesive force of 8.8 MPa, which satisfies the required rock cohesion of greater than 8.7 MPa in the expected period. According to the comprehensive analysis, the new anti-sloughing drilling fluid featured by good inhibition, plugging and anti-sloughing effect, it can solve the problems encountered during Paleogene shale drilling in the Xijiang Oilfield of the South China Sea, and effectively control the borehole enlargement rate.