During the excavation of water-rich cave tunnels, it remains crucial to establish the safe rock pillar thickness at the excavation face to define the required reinforcement measures and guarantee operational security throughout engineering processes. Many factors, including geological conditions, the extent of karst development, and the water pressure within the karst cavity, influence the thickness of the safety rock pillar in a karst tunnel. Utilizing the displacement mutation criterion of the tunnel face, the safe rock pillar thickness for the karst tunnel is quantified through finite element modeling. The methodology commences with a test of the tunnel rock's geomechanical behavior, followed by parameter calibration for the tunnel's geological envelope. Based on the experimental correction parameters, the numerical simulation of the longitudinal deformation characteristics at the karst tunnel face is carried out, ultimately identifying the critical proximity range between solution cavities and working faces that triggers interfacial displacement instabilities. The thickness of the safe rock pillar is computationally determined through displacement mutation criterion at the tunnel face, with systematic investigation revealing the thickness of the safe rock pillar and its variation law across varying geostatic stresses and hydrostatic conditions. The results show that: (1) As submersion duration increases, the surrounding rock's cohesion and internal friction angle typically demonstrate a declining trend, resulting in reduced shear strength of the rock mass. (2) Under deeply buried conditions, the precision of the stratum structure method and its convergence limitations lead to reduced accuracy in determining safe rock pillar thickness when abrupt displacement occurs at the tunnel face, particularly when the karst cavity is positioned 1 m to 4 m from the tunnel face. (3) Integrating numerical modeling with displacement anomaly criteria enhances the accuracy of safe rock pillar thickness calculations during abrupt displacement events at the tunnel face. The required thickness of the protective rock barrier rises proportionally with increased hydraulic pressure within the cavity and the tunnel's burial depth.