Improving the plugging ability of drilling fluid is an effective way to solve the instability of the wellbore in complex formations.Low porosity,low permeability and micro nano scale fracture developed in shale formation.Traditional large diameter plugging materials can not effectively block micro and nano pores,and drilling fluid filtrate is easy to enter the formation,leading to instability of shaft lining.With the help of GCTS equip-ment,we carried out the plugging evaluation experiment of nano CaCO3 plugging agent drilling fluid to the shale cores of the long Ma Xi formation in the Sichuan Chongqing for-mation.It is proposed to evaluate plugging effect by using shale permeability and longitudi-nal and lateral wave velocity characteristics before and after plugging.The results show that under the same concentration condition,the permeability of core decreases and the acous-tic velocity increases with the use of nano CaCO3 plugging agent,which is much better than the effect of base slurry plugging;In the same nano particle material,with the in-creasing content of the nano CaCO3 plugging agent,the permeability of shale is reduced and the acoustic velocity increases.When the content of nano CaCO3 is 3%,the sealing effect of nanoscale drilling fluid is the best.Through the experimental evaluation study,we provide basic experiment and method support for the optimization of plugging agent for pre-venting wellbore instability.