Abstract: Well Ying-1 is an ultra-deep key investigative risk management pre-exploration well deployed by Sinopec in the Shunbei Oil and Gas Field with designed well depth of 9 016.85 m (TVD 8 603.00 m). Drilling the welln encountered downhole problems including wellbore instability, well leakage and borehole wall sloughing. They also easily occur in drilling hard brittle mudstone formations, such as the Silurian Kepingtage Formation and the Ordovician Sangtamu Formation. For this reason, laboratory studies have been carried out to analyze mechanisms contributing to the instability of large section of hard brittle mudstone shale. As such, the reasons for leakage in the Silurian high pressure sensitive fissured formation, and the reasons for instability in Ordovician fractured formations. Based on the basic theory of " multivariate synergistic” wellbore stability, the drilling fluid system SMHP-1 with strong inhibition and sealing capacity was constructed, and the technical measures of mud loss and borehole wall collapse prevention were worked out. The well successfully drilled through a large section of hard brittle mudstone and broken formation to a total depth of 8 588 m without borehole wall instability and drilling fluid loss, setting a record of the deepest onshore well depth in Asia. The field application showed that the drilling fluid system SMHP-1 could effectively solve the problems of wellbore stability and leakage in deep mudstone and broken formations by strong inhibition and sealing capacity, and provide best practices for safe drilling of deep or ultra-deep wells at home and abroad.