To investigate the bearing capacity of the tail brush seal of shields and explore the main factors and failure paths that affect the bearing capacity of the tail brush seal, the principle analysis and simulation test methods are used to comprehensively analyze the components and pressure bearing principle of the tail brush seal and introduce the concepts of "tail brush seal unit" and "tail brush seal system". Next, the difference between the tail grease pressure resistance test and the bearing capacity of a "tail brush seal unit" is analyzed. Furthermore, the main factors affecting the bearing capacity are deduced based on seal failure cases. Finally, a test scheme based on the general working conditions of the tail brush seal is designed. The following are the main results of the study: (1) The main failure of the "tail brush seal unit" is caused by pressure that permeates through seams of the shield tail brush welding structure and spring plate joint and penetrates along the insufficient adhesion between the tail grease and inner wall of the shield structure. (2) A "tail brush seal unit" experiences a pressure of 0.2 MPa; the pressures of multiple "tail brush seal units" can be stacked up, resulting from penetration joints. These results provide a basis for research and design of a "tail brush seal system" with higher bearing capacity.