Abstract: MO-DRILL oil-based drilling fluid was introduced to solve the problems of high difficulty in hole cleaning, frequent downhole complications and difficult reservoir protection during the operation of water-based drilling fluid in East China Sea. The oil-based drilling fluid system suitable for the characteristics of East China Sea is optimized and evaluated indoors. Laboratory evaluation and field application results show that the oil-based drilling fluid has good rheological properties, inhibition properties and reservoir protection properties, and has good application effect in East China Sea. At the same time, in view of the problems of hole cleaning and lost circulation in the application process, relevant measures such as optimizing the rheological parameters of oil-based drilling fluid, enhancing plugging, preventing leakage and optimizing engineering parameters are proposed to further meet the needs of safe and efficient development of oil and gas fields in East China Sea.