Abstract: Tunnel construction in fractured and loose soft rock strata often leads to severe deformation and localized collapses, posing serious threats to construction safety and significantly delaying project progress. The current classification system for highway tunnel surrounding rock and the corresponding support structure design parameters are inadequate to effectively address such deformation challenges. Therefore, research on the optimization of support structures for surrounding rock is urgently needed. This study focuses on a section with Class V surrounding rock in a carbonaceous phyllite tunnel. A 1:80 scale physical model test was conducted, for which a custom-designed model box and measurement system were developed. The study systematically investigated three categories of support structure parameters: thickness of shotcrete, spacing of steel arch supports, and thickness of secondary lining. The test results show that all three factors significantly affect surrounding rock stability. The optimal configuration—30 cm shotcrete thickness, 64 cm steel arch spacing, and 64 cm secondary lining thickness—can effectively control ongoing deformation of the surrounding rock while ensuring the safety and economic efficiency of the support structure.