The safety rock column in front of the working face during the construction of water-rich karst tunnel is crucial to ensure the safe construction of the tunnel. In order to calculate a reasonable safety column thickness at the working face of the tunnel, a three-dimensional fluid-solid coupling numerical model was established based on the plastic zone penetration criterion and displacement mutation criterion in the Yingpangshan Tunnel of the Huali Expressway, and the safety column thickness at the working face was studied under different tunnel depths, water pressure in the cavern and excavation methods, and the interval of the safety column thickness at the working face was given. The results show that: the safety column thickness increases with the increase of tunnel depth and water pressure in the cavern in front of the working face; the smaller the diameter of tunnel excavation and the more timely the support, the smaller the safety column thickness; the minimum safety column thickness interval increases with the increase of tunnel burial depth, but decreases with the increase of water pressure in the cavern in front of the working face. When CRD method is used, the safety column thickness and the safety column thickness interval at the working face are the smallest, and the stability of the working face of karst tunnel is the best.