Abstract: Natural fractures are developed in the deep shale, which are generally micro/nanoscale fractures. However, the existing micro/nanoparticle plugging agent is easy to agglomerate and usually used in a large amount, resulting in downhole failure and complex situations including wellbore instability and sticking. 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. The hydrophobic nano-plugging agent N-MMT was synthesized. It was characterized by infrared spectrum, energy spectrum analysis, particle size analysis, and scanning electron microscope,etc. 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-plugging agent N-MMT has a pre-designed group with an average size of 526 nm, which can significantly reduce the self-imbibition capacity of shale core, and the self-imbibition amount 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 by 47%. In addition, it increases the emulsion-breaking voltage and enhances the stability of the system. Nano-plugging agent N-MMT can adhere to the surface of shale micro and nano pores to form a tight plugging film, preventing drilling filtrate from entering fractures in deep shale and protecting the borehole stability, and has certain application foreground.