Abstract: To minimize environmental pollution and damage to the ecosystem by drilling fluids used in the exploration and development of oil and gas, realize "green" drilling and maximize the performance of a drilling fluid, an encapsulator and a bionic borehole wall strengthening agent previously developed were compounded with other selected additives to formulate an environmentally friendly strongly inhibitive chloride-free water base drilling fluid. Laboratory experimental results show that this drilling fluid has superior temperature stability, resistance to contamination and inhibitive capacity. The drilling fluid had stable properties after continuously hot rolled for 72 h at 120℃. It was resistant to contamination by 10% clays, 0.8% calcium, or 30% salt. Percent shale recovery on hot rolling test was 98.5%. Laboratory evaluation shows that this drilling fluid has performance that is superior to that of PHPA/KCl drilling fluid. This drilling fluid can satisfy the needs of general environment protection and contains no chlorides, minimizing its negative effects on soil structure, plant growth and underground water system. EC₅₀ of this drilling fluid is greater than 10⁶ mg/L and BOD₅/COD_Cr 0.633. The content of heavy metals is far lower than standard requirements, and is thus free of biotoxicity, biodegradable and dischargeable. Experimental data and evaluation results demonstrate that this drilling fluid is a novel mud that both satisfies the needs of drilling operation and protects the environment from being polluted, and has good application prospects.