Abstract: During the exploration and development of G structure in Xihu Sag in East China Sea, the deep strata below Huagang Formation has suffered from many underground complicated situations, such as stoppage of drilling and block of shaft wall caused by shaft wall instability. In this paper, the longitudinal profile of formation pore pressure is calculated by using Eaton method, and it is found that the abnormal pressure is mainly concentrated in the deep strata below Huagang Formation. According to Mohr Coulomb criterion, the main reason for borehole wall instability in this structural area is the mechanical instability caused by drilling fluid density lower than formation collapse pressure and mudstone hydration caused by drilling fluid mud properties. In order to ensure the safety of subsequent drilling operations, it is suggested to carry out pre-drilling pressure prediction research in the relevant block of Structure G, and optimize the oil-base drilling fluid to avoid mudstone hydration.