Abstract: The lithology of Ordovician Sangtamu Formation in Manshen 1 fault zone is composed of mudstone and marl with well developed fractures and high risk of wellbore collapse, which brings great challenges to safe drilling. When Well Manshen 1 was drilled to the depth of 7 392.54 m (Sangtamu Formation), a strike slip fault zone was encountered, resulting in well collapse and sticking of drill tools, which was difficult to deal with. Backfill sidetracking was thus implemented. To solve this, the formation characteristics of Sangtamu Formation were analyzed, and the high-performance anti-sloughing water based drilling fluid technology was developed. Reasonable drilling fluid density was selected to mechanically support the borehole wall, compound fluid loss control measures were adopted to reduce the damage caused by filtrate on the water sensitive mudstone formation, and multi-amino borehole wall inhibitors was introduced while increasing K⁺ content to achieve a multiple anti-sloughing effect. At the same time, the temperature resistance, lubrication property and plugging performance of drilling fluid were improved to meet the requirements of inhibition, plugging, anti-sloughing and high temperature stability of the Sangtamu Formation. The application in the sidetracked borehole of the Well Manshen 1 showed that the drilling fluid performance was stable during sidetracking, the K⁺ content was maintained to be at around 35 000 mg/L, and the high temperature and high pressure filtration rate was reduced from 11.3 mL to 8.0 mL at 150℃. During the whole construction, no borehole instability occurred in the Sangtamu Formation. The well was successfully drilled to the total depth of the third spud, and the casing was set in place in one run. This showed that the anti-sloughing effect of high performance water based drilling fluid was remarkable, and it achieved the expected goal.