Cured-in-place pipe (CIPP) is commonly used to rehabilitate deteriorated culverts and drainage structures. During installation, factors like corrosion, joint staggering, and insufficient inflation can create an annular gap between the liner and host pipe, potentially leading to buckling failure under external loads. Existing design methods lack experimental evaluation, leading to discrepancies between simulations and monitoring. To address this, an experimental study was conducted using a carefully designed test system to minimize boundary condition influences. Two tests were performed with host pipes of 600 mm and 1000 mm outer diameters. Simulating underground water pressure, elliptical deformation of the CIPP liner was observed until the host pipe’s wall was contacted, with localized arching in non-contact areas. The width of the annular gap (Δd) between the liner and host pipe determined whether hydrostatic buckling occurred before critical buckling. However, the critical buckling pressure could not be accurately predicted by existing buckling models. In response to this issue, an improved model is proposed based on pressure from elliptical deformation and buckling caused by local arching. The experimental results showed that the simulation error ranges from -0.5 % to +1.1 %. Our study provides valuable insights into enhancing CIPP liner buckling resistance and offers a more accurate model for predicting critical buckling pressure, contributing to safer and more reliable CIPP rehabilitation methods.