The waterproofing of shield tail sealing system is an important factor affecting the safety of shield construction. A laboratory test of the shield tail sealing system was designed, and a corresponding simulation model was established based on the computational fluid dynamics to analyze the characteristics of waterproofing failure. The changes of volume distribution, pressure field and velocity field of two-phase fluid in the process of external hydraulic intrusion were analyzed, and the influence of shield tail structure parameters, tail brush parameters and grease parameters on seal performance was studied. The results show that: 1) increasing the number of shield tail brushes and the filling rate of shield tail brush steel wire can effectively improve the waterproofing ability; 2) In the process of waterproof failure, the peak point of flow velocity appears at the tail brush of shield, and the expansion distribution of flow velocity is consistent with the volume distribution of water. 3) The permeability of shield tail brush, the viscosity of grease and the exposed length of wire bundle are the sensitive factors affecting the initial leakage time. By reducing the permeability of shield tail brush, increasing the viscosity of grease and reducing the exposed length of wire bundle, the initial leakage time can be delayed, and the waterproof ability can be effectively improved.