The stability of the surrounding rock and outburst prevention thickness of karst tunnel are not invariable. Still, it is greatly affected by the size of karst tunnel, and the influence law and mechanism are not clear. Based on FLAC3D numerical simulation method, the three-dimensional calculation model of tunnel karst cave and the surrounding rock energy visualization program are constructed, and the influence of karst cave size on the mechanical response and energy evolution law of tunnel surrounding rock is systematically studied. The results show that: The numerical results are in good agreement with the actual monitoring results, which verifies the reliability of the model; After tunnel excavation, the ratio of karst cave radius to tunnel span (T) has a great influence on the stress of surrounding rock of tunnel, and the degree of tensile stress concentration is positively correlated with the size of karst cave; With the increase of T, the distribution range of displacement is expanded, but the stress is released through the buffer space, resulting in the decrease of the maximum settlement and uplift value; The large karst cave makes the energy release range expand but the intensity decreases, indicating that the stress disturbance is more dispersed; With the increase of T, the minimum outburst prevention layer thickness gradually increases, but the increase amplitude gradually decreases, and gradually tends to saturation. The research results can provide theoretical reference for tunnel stability control in karst area.