Nano pores and micro fractures are well developed in shale formation. However，the particle size of conventional plugging agent is large and it is difficult to plug pores and fractures in shale. A core-shell nano plugging agent（CLG-NM）with high temperature resistance was prepared by using inorganic nano silica as the core and poly（styrene-butyl acrylate-acrylic acid）as the shell in mass ratio 10∶2∶1 through emulsion polymerization method. The plugging agent was characterized by infrared spectroscopy， transmission electron microscopy，dynamic light scattering and thermogravimetry，and its plugging performance was evaluated by shale pressure transferring experiment. The results showed that the particle size distribution of CLG-NM was 40—300 nm，and the median particle size was 89.4 nm. CLG-NM maintained good thermal stability below 372 ℃. Furthermore，CLG-NM had good compatibility with oil-based drilling fluid. 3% CLG-NM had little effect on the fluidity of oil-based drilling fluid. The high temperature and high pressure filtration decreased from 3.1 mL to 2.8 mL after aging at 180 ℃ for 16 h，and the demulsification voltage kept higher than 700 mV. CLG-NM could enter the nano pores and fractures in the shale for plugging. The upstream pressure of oil-based drilling fluid contained CLG-NM could penetrate the core for 19.5 h. Compared with similar products at home and abroad，CLG-NM had better plugging effect on nano pores and fractures in shale.