To address the engineering challenge of surrounding rock instability in karst tunnels induced by concealed cavities, a macro-micro coupled analysis method was established by integrating visualized physical model tests with Particle Flow Code (PFC) discrete element simulations. The proposed method was validated through its application to the Shenzhen Metro Line 3 project. The results indicate that the underlying karst cave alters the stratum structure and induces stress redistribution in the surrounding rock, leading to shear slip, loosening, and block detachment at the horizontal diameter of the cave. Tunnel excavation triggers secondary stress redistribution, and under the influence of the soil arching effect, the horizontal diameter ends of the tunnel become the weakest zones. As excavation advances, shear failure occurs in the tunnel floor, which eventually connects with the cave, while the slip surface propagates upward from the weakened zones to the ground surface, ultimately forming a funnel-shaped collapse. At the mesoscopic level, the karst cave induces a "cavity effect," disrupting the force chain transmission path and breaking the original "displacement–stress" coordinated balance of the surrounding rock. The stress concentration zone above the cave overlaps with that at the tunnel arch foot, and a stress "blank zone" emerges between the tunnel and the cave, further reducing the bearing capacity of the surrounding rock. The research findings reveal the progressive failure mechanism of karst tunnel surrounding rock from both macro and micro perspectives, providing a scientific basis for disaster mechanism studies and construction optimization in similar concealed geological hazard scenarios.