<p>To explore the deformation response of shield tunnel segment linings under surface surcharge in composite strata, a self-developed "shield segment stratum–structure method loading system" was employed to conduct a scaled model test. The convergence deformation and failure processes of shield tunnel segment linings across composite strata with varying hard rock ratios were analyzed, revealing structural instability and failure mechanisms. A numerical model was also constructed to evaluate trends in ovality and load, leading to a fitted equation between the hard rock ratio and instability ovality. The results indicate that the proportion of hard rock significantly influences the damage characteristics and load-bearing capacity of tunnel segments, with damage primarily located at soil–rock interfaces and segment joints. An increased hard rock content improves tunnel stability and peak stiffness; however, as the hard rock ratio increases from 0 to 1, the instability ovality decreases from 36.1‰ to 7.0‰, and the damage ovality decreases by nearly 5 times, both of which are inversely related to the proportion of hard rock. Higher rock ratios result in greater instability and failure loads, with slower initial deformation. Damage rapidly escalates once the instability load is reached, leading to brittle failure. Therefore, early reinforcement is more beneficial for tunnel segments with higher proportions of hard rock.</p>