The elderly water supply pipelines rehabilitated with cured-in-place-pipe (CIPP) technology can withstand internal water pressure loads. However, the existing pipeline’s hole defects provide seepage channels for groundwater around the pipe, leading to a significant increase in local static water external pressure and causing instability and failure of the lining. Through static water external pressure tests and finite element analysis of two full-scale pipeline groups, DN 600 and DN 1000, this study investigates the deformation patterns and instability characteristics of the cured lining under the influence of static water external pressure. It explores the influence of lining DR values and ellipticity on the buckling behavior of the inner lining under interface bonding conditions. The results indicate that: The inner lining undergoes five stages under static water external pressure: uplift, bending, ultimate external pressure buckling, local fiber fracture, and lining fracture. The bonding strength of the interface significantly affects the buckling of the lining. When repairing water supply pipes using CIPP technology, it is essential to ensure the durability of the bonding performance and bonding effect between the CIPP lining and the existing pipeline.