Abstract: The target zones in the block Shuangyushi in west Sichuan are buried deep, and wells drilled in that block have completion depth of at least 7 000 m. Air drilling is generally adopted in drilling the upper φ444.5 mm and φ333.4 mm in an effort to drill fast. It is critical to convert air drilling fluid to liquid base drilling fluids, and so is the performance of drilling fluids used in drilling with "high efficiency PDC bit + PDM". KCl organic salt polymer drilling fluid, under the synergistic effect of KCl and organic salts, performs much better in inhibiting the hydration, dispersion and swelling of shaly formations. Plugging agents, on the other hand, quickly form dense mud cakes on the borehole wall, thereby reducing invasion of mud filtrate into formations and preventing the dry borehole wall generated in air drilling from collapsing because of water (mud filtrate) absorption. By minimizing hydration and dispersion of drilled cuttings, mud rheology becomes stable and is easy to maintain. A drilling fluid with good inhibitive capacity, low viscosity and gel strength is helpful in enhancing bit hydraulics and minimizing adsorption of drilled cuttings on drill bit, thereby avoiding bit balling. Reduction in the concentration of micron-sized particles and submicron-sized particles is helpful in ROP enhancement. Field application of the drilling fluid in well Shuangtan-8 showed that the use of KCl-organic salt polymer drilling fluid ensured the borehole stability of the air drilling section, helped drilling through the bottom of the Shaximiao formation (Shayi member), Lianggaoshan formation, Ziliujing formation and Xujiahe formation successfully. During drilling operations, tripping of drilling string was smooth, borehole wall was stable, electric wireline logging was conducted favorably, no downhole problems took place, and penetration arte was satisfactory. The use of the KCl-organic salt polymer drilling fluidhas satisfied the needs of engineering and mud logging.