Abstract: During the drilling of horizontal sections of horizontal shale oil wells in the Cangdong and Qikou sags of the Dagang Oilfield, technical difficulties such as wellbore instability, poor borehole cleaning, and high friction and torque were encountered. Based on analysis of geological characteristics of shale formation, technical countermeasures were made to enhance the inhibition, plugging, and rock-carrying properties of drilling fluids. Key treating agents including plugging agents and lubricants were optimized to form the BH-KSM-Shale and BH-WEI-Shale high-performance water base drilling fluids which possess good inhibition and plugging abilities. The performance evaluation results show that the drilling fluids have good inhibition performance, rock carrying performance and plugging performance. The developed drilling fluids can decrease the shale permeability, resist the pressure transfer, and ensure the wellbore stability. The BH-KSM-Shale and BH-WEI-Shale drilling fluids were applied to the drilling of the horizontal sections of 36 horizontal shale oil wells in the Dagang Oilfield. As a result, the average hole diameter enlargement rate of the horizontal sections was 6.8% and no downhole failure related to drilling fluids occurred. These results indicate that BH-KSM-Shale and BH-WEI-Shale drilling fluids can solve the difficulties in the drilling of horizontal sections of horizontal shale oil wells in the Dagang Oilfield, and provide technical support for the drilling of horizontal shale oil wells in the Dagang Oilfield.