Abstract: To deal with the scaling and sloughing of rigid and fragile shale in Mesozoic and Palaeozoic group of Well Bo 930,the non-invasive drilling fluid was selected.The borehole instability problems of Mesozoic and Palaeozoic group in this area have been analyzed,and main reasons of formation sloughing was founded which are caused by thick illites hydration shell,great hydration repulsion,and abundance in cracks.The formulation of non-invasive drilling fluid was developed,and through adding non-invasive additives which is a mixture of a series of oil-soluble and water-soluble modified polymer,anti-sloughing measures can be strengthened.Different maintenance outlines have been made according different formations.Field application shows that the non-invasive drilling fluid system used in tertiary drilling can rapidly form plugged zone with ultra low permeability and prevent fluid pressure enterring into formation,so as to prevent crack shale formation sloughing and scaling of this area.The probleums of sloughing and scaling of this area has been solved successfully through the utilization of non-invasive drilling fluid,so this drilling fluid can be popularized in this area furthermore.