Abstract: Conventional sulfonated fluid-loss reducers show poor stability under ultra-high temperatures, are prone to degradation or cross-linking that leads to rheological deterioration, and difficulties in fluid-loss control. To address these issues, key chemicals such as the non-sulfonated polymer POIY-XL and modified humic acid POIY-JL were developed, and a high-temperature-resistant high-density non-sulfonated water-based drilling fluid was formulated. Laboratory performance evaluations showed that POIY-XL exhibited excellent performance in high-temperature resistance, salt resistance, and fluid loss reduction. After aging at 200 °C in saturated brine for 16 hrs, its viscosity retention ratio reached 78.6%, and the H₂S detection value measured immediately upon opening the aging tank was less than 15.2 mg/m³. POIY-JL did not contribute to drilling fluid thickening at high temperatures, however can improve filter cake quality, demonstrating a good fluid loss reduction effect. The formulated drilling fluid system exhibited good rheological properties and strong temperature resistance. After standing at a high temperature for 7 d, the sedimentation factor was less than 0.52, and the filter cake was compact. The high-temperature high-pressure fluid loss at 220 °C was less than 15 mL. The high-temperature-resistant high-density non-sulfonated water-based drilling fluid was tested in the third drilling section of Well Shunbei X in Xinjiang. The bottom-hole temperature of this well was 212 °C, and the actual drilling fluid density ranged from 1.50 to 1.63 kg/L. While drilling, the drilling fluid maintained good high-temperature high-pressure rheological properties and sedimentation stability, with no high-temperature sedimentation observed. A core recovery of 100% was achieved in the interval of 8 690–8 696 m. This drilling fluid achieves the goal of “desulfonation” of high-temperature-resistant drilling fluids while ensuring safe drilling in ultra-deep wells, providing a technical reference for the development of high-temperature-resistant non-sulfonated drilling fluids.