Shield tail sealing system is one of the important systems of shield. When it breaks down, it will cause major construction risks such as water leakage and slurry leakage. It is of great practical significance to study the sealing performance of shield tail sealing system under different external water pressure conditions (2~8 MPa). A multiphase flow numerical simulation is performed on shield tail sealing system using computational fluid dynamics method based on the characteristics of space configuration of shield tail seal chamber and the bidirectional pressure experiment. The simplified method of shield tail brush model, the pressure distribution law of shield tail grease chamber, and the flow law of water invasion are studied. The simulation results show that: (1) after the grease injected becomes stable, the pressure in grease chamber is in the range of 0.8~1.2 MPa, and the pressure in grease chamber 2 is always greater than that in chamber 1 with a pressure difference of 0.1~0.3 MPa; (2) when the water completely breaks through the shield tail sealing system, a stable flow channel will be formed; and (3) with the increase of water pressure, the time needed for completely breaking through the shield tail sealing system gradually decreases, and the amplitude reduction gradually decreases. The fluid simulation performed can provide theoretical guidance for the design of the shield tail sealing system.