Abstract: In order to meet environmental protection requirements for deepwater drilling, an environmently friendly BIO-OIL base fluid was developed for synthetic based drilling fluid. The study compared the BIO-OIL base fluid with commonly used base fluids and examined their basic physical and chemical properties, viscosity temperature characteristics, carbon atom number distribution, biotoxicity, and micro-emulsion rheology. These studies studied the BIO-OIL based deepwater synthetic based drilling fluid and HTHP synthetic base drilling fluid and evaluated their performances. The results showed that the basic performance of BIO-OIL could satisfy minimum requirements for field application, while also satisfying the requirements of Class I sea discharge bio-toxicity level requirements. The type of emulsion fluid is the same as that of 3^# white oil emulsion, with the 3D network structure, and it has the advantage that it can be made into deepwater synthetic based drilling fluid and HTHP synthetic based drilling fluid and it meets operational requirements. The BIO-OIL based synthetic drilling fluids have been tested in three wells in western block of South China Sea. Their rheological properties were stable, its cuttings carrying performance was strong, and it demonstrated good wellbore cleaning effect. Studies indicated that BIO-OIL can be classified as an environment friendly base fluid, which also meets operational standards for synthetic based drilling fluids.