Abstract: The shield tunnelling method has become the mainstream construction one for urban subway tunnels. Due to the poor sealing effects of shield tail brushes, many severe safety accidents such as water and slurry leakage occur frequently. The influences of grease consistency, grease pressure and shield tail clearance on the watertightness performance are discussed by setting up the sealing unit test devices for the shield tail brush-grease chamber. The test results show that: (1) According to the different grease cone penetrations, there are two modes of shield tail sealing leakage. One is that the leakage material is the external slurry at low cone penetration, and the other is that the leakage material is the external slurry and grease mixture at high cone penetration. In order to improve the shield tail sealing performance, the grease cone penetration should be controlled within the range of 180~200 (1/10 mm). (2) The critical leakage pressure of the shield tail can be determined by the grease pressure and adhesion force, which is slightly smaller than the grease pressure. Therefore, it is reasonable to set the grease pressure 0.5 MPa higher than the slurry pressure in shield tail clearance during shield tunneling. (3) The critical leakage pressure of the shield tail is more affected by the grease pressure, but the adhesion force of the shield tail brush affects the grease escape and grease pressure stability. Hence, the management of the shield tail brush and sealing system during shield tunnelling should be strengthened to avoid the tail sealing failure, which is caused by the loss of elasticity of the shield tail brush due to mortar intrusion. The research results may provide reference for the design and construction management of the shield tail sealing system.