The pressure dissolving cavity in karst area is often disadvantageous to the safety of tunnel construction. For the study of karst cavity effect on the construction process of the tunnel surrounding rock and the supporting force, relying on a system of karst tunnel, using Flac3d finite element method to carry out numerical experiments on different conditions, we mainly considered the cavity size, cavity position and cavity pressure effects of the three factors, and analyzed the surrounding rock deformation under different conditions and the stress of surrounding rock and supporting force. The results show that the karst tunnel cavity will cause the deformation of surrounding rock convergence of tunnel construction process and stress increase. The cavity size, cavity position, and cavity pressure can affect the deformation converge and cavity pressure. The greater the cavity will cause near the convergence point and deformation value increased stress. Cavities located at the vault of the tunnel had the greatest impact, followed by the cavities located in the side wall of the tunnel effect. The cavity located in the arch of the tunnel cavity has a minimal impact. The existence of cavity affect the initial supporting force caused by cavity proximal early stress redistribution; therefore, we recommend that to strengthen the primary overall support strength of the cavity construction section.