Abstract: To overcome the drawbacks of conventional sulfonated fluid loss control agents, which suffer from poor thermal stability at ultra-high temperatures and are prone to thermal degradation or crosslinking, resulting in deteriorated rheology and difficult filtration control of high-density drilling fluids, key reducers such as non-sulfonated POIY-XL and modified humic acid POIY-JL were designed and developed, and a non-sulfonated high-temperature resistant high-density water-based drilling fluid suitable for deep formation drilling was constructed. Laboratory performance evaluation shows that POIY-XL has good resistance to high temperature, salt resistance and filtration reduction effects. After aging in a saturated saltwater environment at 200℃ for 16 hours, the viscosity retention rate reached 67%, and the H₂S detection value at the moment of opening the aging tank was less than 15.2mg/m³. POIY-JL does not cause thickening of drilling fluid at high temperatures, can improve the quality of mud cake, and has good filtration reduction effect.The constructed water-based drilling fluid system with a density of 2.1g/cm³ has good rheological properties, with less than 15mL at 220℃ of high temperature and high pressure filtration loss and less than 0.52 of settling factor after 7 days of high temperature standing.The non-sulfonated high-temperature resistant high-density drilling fluid was applied in the third well of Shunbei Xin Xinjiang. The temperature at the bottom of the well is 212℃, and the density of the actual drilling fluid is 1.50-1.63 g/cm³. During the drilling process, good high-temperature and high-pressure rheological properties and settlement stability were maintained, with no high-temperature settlement occurring. The core recovery rate in the 8690～8696m well section reached 100%.This drilling fluid has achieved the goal of "desulfonation" of high-temperature resistant drilling fluid under the premise of safe drilling in ultra deep wells, providing technical reference for the research and development of high-temperature non-sulfonated drilling fluids in China.