Abstract: In order to improve the temperature stability and environmental protection performance of a high-temperature resistant and high-density drilling fluid system, the self-developed filtrate reducers, inhibitors, and lubricants with biomass synthetic resin were taken as the core treatment agents, the dosages of the treatment agents were optimized, and a high-temperature resistant and high-density biomass drilling fluid system was constructed. The performance evaluation results showed that the system could resist temperature up to 200 °C. It also could resist CaCl₂ pollution of 1%. The rock cuttings rolling recovery was 94.3%, with the lubrication coefficient ≤ 0.128. The biological toxicity EC₅₀ value was 89 230 mg/L. The field application showed that the high-temperature resistant and high-density biomass drilling fluids could significantly resist pollution. They had excellent rheological stability when the density was up to 2.55 kg/L and the bottom hole temperature was 140 °C. The high-temperature resistant and high-density biomass drilling fluids can promote the utilization of biomass resources in the drilling fluid field and solve the trade-off between the temperature resistance and environmental protection of high-density water-based drilling fluids. Therefore, the drilling fluids have positive field promotion and application value.