Abstract: Well SHB1-6H is an ultra-deep well drilled in Shunbei area by Sinopec, and is the first well in this area to use 4 casing string sizes. The second interval of the well, with open hole length of 4,460 m, 1,463 m of which is mudstones, has complex geology; The Trias, Carboniferous and Silurian systems are mainly hard and brittle mudstones, apt to cave-in. Lost circulations have been easy to occur in drilling the shattered Permian system. To avoid these problems, a high temperature salt-resistant filter loss reducer, RHPT-1 and a filming agent, CMJ-1, were used in the potassium- and amine-treated sulfonated polymer drilling fluid to improve the inhibitive capacity, plugging performance and filtration behavior. In field application, the improved polymer sulfonate drilling fluid had rheology easy to control. To maintain the inhibitive capacity of the drillingfluid, 5-7 kg/m³ of HPA (an amine based shale inhibitor) and 30-50 kg/m³ of KCl were kept therein. A high softening point asphalt was used to enhance the plugging performance of the drilling fluid, and to reduce the HTHP filter loss to less than 10 mL. With these measures, borehole instability in the formations below the Trias system was effectively solved. In drilling the Permian system, bamboo fibers, ultra-fine CaCO₃ and polymerized gel LCM have been used to avoid mud losses. The hole enlargement for the second section of well SHB1-6H was 12%, reduced by 63.74% than the adjacent well.