Abstract: Considering the abnormal high pressure of CNOOC Myanmar blocks, technical research of ultra-high density water-base drilling fluids is conducted. The technical difficulties and the influence mechanisms of rheological properties and filtration of ultra-high density water-base drilling fluids are analyzed. The regulation ideas of the properties are proposed. The effect of the organic salt weight3 for high-density drilling fluid properties is studied. The fluid loss agents and blocking agents in the system of organic salts and free-clay are optimized. A kind of water-base drilling fluids system is developed and evaluated, with the density being 2.80 g/cm3, apparent viscosity less than 100 mPa?s, and filter loss less than 15mL in HTHP. Evaluation shows that the system has good thermal stability and it can be resistant to 160 ℃. Besides it has good dynamic stability. The density difference of the system between the upper and lower drilling fluid is 0.03 g/cm3, when it is kept at room temperature for 24 hours. It also has strong anti-pollution capability and can be resistant to 5% NaCl, 1% gypsum and 5% poor-quality soil. So the system could meet the requirement of field application for its comprehensive performance.