Abstract: For the waterproof safety of undersea shield tunnels with high water pressure, the contact stress and waterproof capacity of T-joint are studied considering the background of Qiongzhou Strait Tunnel. Firstly, a fluid-solid coupling model for the T-joint is established to reveal the distribution characteristics of gasket-gasket contact stress and the seepage pattern. Then, a series of waterproof laboratory tests of the T-joint are conducted using the servo-type equipment developed independently to obtain the critical water pressure of joint leakage. Thirdly, the formula for the critical water pressure of leakage varying with the joint opening is derived based on the numerical data and the in-laboratory test results. The results show that the maximum value of the average contact stress is located at the corner of the T-joint, and the average contact stress of the location 20 mm away from the T-joint corner on the longitudinal seam is the lowest where the leakage is likely to occur. The seepage pattern is consistent with the distribution of the average contact stress, and the seepage velocity is greater where the average contact stress is lower. The critical water pressure of the T-joint is negatively related to the joint opening, and the joint opening should be lower than 10 mm to ensure the long-term waterproof safety of the Qiongzhou Strait Tunnel. The maximum error of the formula for the critical water pressure of leakage is 15.7%, meeting the accuracy requirements.