To study the influence of the overlying sand layer collapse induced by the karst cave on the existing shield tunnel, an equivalent simulation method based on stratum parameter reduction is proposed, and the methods of the stratum parameter reduction range and coefficient are also determined. The surface settlements and deformations of existing tunnels when the collapses of the overlying sand layer induced by karst caves with different occurrence conditions and scales are analyzed based on Wuhan metro line 11. The research results indicate the following: (1) The collapse of the sand layer over the karst cave will increase the stratum deformation of the existing shield tunnel. When the karst cave is located directly below the tunnel, the surface settlement curve presents a V shape, whereas when the karst cave is located below one side of the tunnel, the maximum ground settlement deviates from the location where the karst cave occurs. (2) The collapse of the overlying sand layer on the karst cave causes a significant increase in the compressive stress of the tunnel lining structure at the top and bottom of the tunnel, and the largest increment occurs at the bottom of the tunnel. With an increase in the scale of the cave, compressive stress of tunnel lining structure increases gradually at the bottom of arch, but the increase gradually decreases; when the karst cave is located directly below the tunnel, the impact of the overlying sand layer collapse on the tunnel structure stress is more significant.