Hydrologic and engineering geological conditions of underground engineering and mining are highly complex. Geological hazards, especially water inflow or inrush, often occur during constructions. In order to get an insight into flow state evolution laws of water inflow and inrush in karst regions, as well as the influence of tunnel excavation, a numerical simulation method was proposed with COMSOL Multiphysics. Three water flow laws, Darcy’s law, Brinkman equations and incompressible Navier-Stokes equations, are coupled and linked with each other. Water pressure and velocity can be analyzed in the course of excavation, and it can be used for further study of risk control of water inflow and inrush.