In order to improve the wellbore instability caused by poor plugging performance of drilling fluid during the drilling of shale oil and gas horizontal wells，firstly，the composition and the wellbore instability of Longmaxi shale in South Sichuan were analyzed. Then n-cetyl-triethoxy-silane，aminopropyl-triethoxy-silane and activated carbon nanotube were used as raw materials to synthesize the nano-micron plugging agent（NP-1）for oil-based drilling fluid. Its structural and physicochemical properties were analyzed by Fourier transform infrared，thermogravimetric，transmission electron microscope and surface wettability，and then its compatibility with oil-based drilling fluid was investigated. Finally，the plugging performance was evaluated by breakthrough pressure，pressure transmission and triaxial compressive strength. The mechanism was analyzed and then the field application was carried out. The results showed that there were lots of developed pore throats in size of nanometer and micron-meter and serious capillary spontaneous imbibition of shale in South Sichuan. As a result，the liquid would permeate into the wellbore continuously， which would lead to wellbore instability. The diameter of NP-1 was in the range of 30—50 nm，the length was in the micron-meter， the surface wettability was hydrophobic and lipophilic，and then the thermal stability was good below 385.2 ℃. NP-1 had good compatibility and stability with oil-based drilling fluid. The high temperature and high pressure filtration loss after aging 16 h at 180 ℃ was reduced from 2.8 mL to 1.8 mL after adding 3% NP-1 to ordinary oil-based drilling fluid. The oil-based drilling fluid with NP-1 could effectively plug the end face of shale，which could improve the breaking pressure，prevent the pressure transmission，stabilize the core internal structure and compressive strength，and then maintain the wellbore stability. Field application results showed that NP-1 could effectively alleviate the lost circulation and wellbore instability of adjoining well caused by the weak plugging ability of drilling fluid. The enlargement rate of well diameter in average was only 5.61% after treating. NP-1 had excellent plugging performance for shale in oil-based drilling fluid. It provided a reference for efficient drilling of similar complex shale gas wells in South Sichuan.