Abstract: There are a large number of fractures in the deep shale, which are generally micro/nano scale fractures. However, the commonly used micro and nano particle plugging agent is easy to agglomerate and the amount is huge, resulting in downhole instability and stuck drilling.In order to solve this problem, nano-montmorillonite was prepared from layered montmorillonite, and then the surface of nano-montmorillonite was modified by atomic transfer radical polymerization, and the hydrophobic nano-sealing agent N-MMT was synthesized.It was characterized by infrared spectrum, energy spectrum analysis, particle size analysis and scanning electron microscope.The plugging performance of N-MMT was evaluated by core imbibition experiment, microporous filtration membrane filtration test and high temperature aging test.The experimental results show that the nano-sealing agent N-MMT has a pre-designed group with an average size of 526nm, which can significantly reduce the self-imbibition capacity of shale core, and the self-imbibition capacity of shale core is decreased by 66.67%. N-MMT has good compatibility with conventional treatment agents for oil-based drilling fluid, which reduces the high temperature and high pressure filtration loss of drilling fluid system by 47%, increases the emulsion breaking voltage, and enhances the stability.Nano plugging agent N-MMT can adhere to the surface of shale micro and nano pores to form a tight plugging film, prevent drilling filtrate from entering deep shale fractures, and help protect the stability of well wall.