The factors leading to the leakage accident of the shield tail of a tunnel project are determined by designing a suite of simulation test devices for loading of the shield tail brush under different conditions. The working states of loading deformation and failure modes under different shield tail clearances are investigated. The results reveal that (1) for a given pressure, the antideformation capacity of the shield tail brush decreases with increasing shield tail clearance. (2) When the clearance is small, the brush can withstand large loads without damage. When the clearance of the shield tail increases to a certain extent, a small load can cause a large deformation of the steel plate comprising the brush. This leads to plastic deformation of the plate or root fracture. Furthermore, the damaged wire of the shield tail brush evaginates, leading to sealing failure of the shield tail.