Abstract: Objectives Ring-stiffened cylindrical shells are widely used as pressure hull structures in underwater equipment, and initial geometric imperfections are inevitably introduced during manufacturing. This study aims to investigate the effects of different types of initial imperfections on the structural response of such shells subjected to underwater explosion loads. Methods A typical titanium alloy ring-stiffened cylindrical shell is selected as the research object. A baseline underwater explosion response is first established for an ideal structure using the acoustic-structure coupling method. Based on linear buckling mode analysis, the local and global instability modes are extracted to represent the initial imperfections of the shell plate between stiffeners and the initial out-of-roundness of stiffeners, respectively. Systematic simulations are conducted with varying imperfection amplitudes to evaluate their influence on the underwater explosion structural plastic response. Results Under identical impact factors, the maximum plastic strain increases linearly when the shell plate imperfection amplitude ranges from 0.2 to 1.0 times of the shell thickness. In contrast, when the stiffener out-of-roundness exceeds 0.6% of the cylindrical shell radius, the maximum plastic strain exhibits a sudden increase and surpasses the material fracture strain, leading to structural failure. Conclusions The initial out-of-roundness of stiffeners is significantly more sensitive than shell plate imperfections in affecting the structural response under underwater explosion loads, and should therefore be strictly controlled during manufacturing.