Abstract: The shield tunnellings are prone to assembly errors during construction that may induce joint leakage, especially at the corners. In this study, a numerical installation model of the T-joint gasket was established to investigate the mechanical behaviour and waterproofing performance of gasket corners under two typical assembly conditions: assembly error of the circumferential joint and insufficient insertion space. The vulnerable leakage location of the gasket at the corner, the extent of corner effect and the mechanisms of corner leakage were revealed through numerical simulation. The results indicate that the circumferential joint is prone to leakage. The assembly forces of circumferential and longitudinal joints should be independently controlled to ensure adequate pre-compression stress of the gaskets. The corner effect of the gasket easily results in the formation of the contact cavity near the corner, leading to a decrease in waterproofing performance. Enhancement measures such as optimizing the corner structure are required to strengthen its waterproofing performance. As the joint opening and offset or the remainder at the corner increases, the waterproofing performance of the gasket at the corner gradually decreases, and the degree of weakening also continues to increase. In practical engineering, it is recommended that the circumferential joint opening be limited to less than 6 mm and the remainder at the corner be controlled below 4 mm, to ensure the waterproofing performance of the corner gasket.