Abstract: In order to solve the complex downhole problems such as difficulty in back reaming, compression torque, logging resistance, and sticking in the operation of grey green complex mudstone sections in the upper part of Longjing Formation and Huagang Formation in some blocks of East China Sea, the mechanism of wellbore instability was studied by analyzing the rock fabric characteristics, physical and chemical properties and mechanical properties of complex mudstone formations. This paper analyzes the influence law and degree of tripping resistance under different drilling fluid properties and drilling conditions, and provides a theoretical basis and foundation for treatment measures for complex situations on site, and forms an optimized field oil-based drilling fluid performance and construction technology. After the practical application of the related technology in the field of complex mudstone formation, the borehole stability during drilling in the complex mudstone formation of Longjing Formation and Huagang Formation was gradually realized, and the tripping time was increased by 96.3%～353.1%, which significantly improved the operation time and reduced the drilling cost. The problem of safe and efficient drilling in the complex mudstone formation in East China Sea was successfully solved, which provided a reference for drilling in similar strata.