Abstract: Oil-based drilling fluids deployed for drilling of used in horizontal well drillings in China are characterized by their low shear strength,poor rheological properties and difficulty in subsequent treatment of cuttings disposal. Under such circumstances,it is necessary to improve their performance and environmental protection features. To meet the demands related to the development of environment-friendly high-performance drilling fluids,natural vegetable oil was used to generate a biomass-based liquid,PO-12,through biologic and chemical modification. Test results showed that the biodegradable PO-12 has outstanding performances in viscosity,heat resistance,stability,inhibition,hydrolysis resistance and fluorescence. With no aromatic compositions and being non-toxic,the environment-friendly PO-12 can satisfy the demand of for drilling of horizontal wells drilling and offshore operations. On the basis of the PO-12,a biomass synthetic drilling fluid system has been developed. With performance equivalent to that of oil-based drilling fluids,the innovative system may provide a new way to enhance environmental protection performances of oil-based drilling fluid systems.