<p>Currently, the reinforcement design of shield tunnel secondary linings mainly depends on engineering experience, with a lack of clear guidance from relevant codes and literature. Relying only on experience during construction can cause structural flaws and safety hazards. This study, based on the Guangzhou–Shenzhen–Hong Kong Shiziyang Tunnel project, uses model tests to study shield tunnel double-layer lining structures. It compares and analyzes the mechanical features and interaction mechanisms of reinforced and unreinforced secondary linings. Results show that in such structures, segmental linings bear the main load, and secondary linings offer extra support and adjust deformation. Reinforcement in secondary linings affects the constraint on segmental linings. Reinforcement enhances overall performance significantly. Although it has little impact on ultimate bearing capacity, it prolongs the load-bearing process. Specifically, it increases the ultimate bearing capacity of segmental and secondary linings by 21.2% and 26.1%, respectively. For 10-m-diameter shield tunnels, secondary lining reinforcement design should be adopted when the equivalent overburden thickness at the tunnel crown exceeds three times the tunnel diameter.</p>