Based on the tunneling process of urban subway shield construction, the aim is to prevent the risk of water and mud inrush faced by subway shield construction in cities where underground Karst has developed and there is a fault fracture zone. Aiming at the critical safety distance of the fault fracture belt in front of the shield tunnel during the construction of the urban subway, the influence law and response sensitivity of different types and diameters of shield machine cutterheads, and different thicknesses and water pressures of the fault fracture belt on the critical safety distance are studied .The COMSOL Multiphysics software fluid-structure interaction method is used to simulate the shield propulsion process and the moment of water inrush. Based on the 3D finite element numerical simulation results, the critical safety distance formula of the fault fracture zone in front of the shield tunnel is obtained by using the multiple regression function in SPSS Statistics software. The results of this paper have certain construction guidance significance for the development of fault fracture zones and the excavation of urban subway shield tunnels with abundant groundwater.