Abstract: Water inrush frequently occurs at karst tunnel face because there is no supporting countermeasure over here. In addition, the catastrophe can lead to serious consequences on account of tunnel face which is as the working face of tunnel project. During the process of excavation, safe thickness of surrounding rock mass between tunnel face and front karst structure is very important and determines whether water inrush happens. The affecting factors of safe thickness of surrounding rock mass are analyzed, and quality of rock mass and karst water pressure are regarded as the dominant factors. Three-dimensional finite elements method is utilized to simulate the process of tunnel excavation. As a consequence, the correlations between safe thickness of rock mass and its dominant factors are figured out. Based on non-linear regression method, the forecasting model for safe thickness of rock mass of karst tunnel face is put forward and verified by two typical case studies. A powerful tool is provided for accurately forecasting the safe thickness of rock mass, even preventing the water inrush in karst tunnel.