T-joints are the weak points in the waterproofing of shield tunnels. This paper focuses on a full-scale experimental and numerical study of the waterproof performance of T-joints in a 6.6- meter diameter shield tunnel. First, a full-scale test platform was independently built, and a waterproof performance test of T-joints was conducted, obtaining the failure pressures at the joints under different dislocations and openings of the circumferential and longitudinal joints. Secondly, based on the test, a three-dimensional finite element model of the elastic gasket at the T-joint was established to study the waterproof performance of the gasket, with mutual verification between experimental and numerical results. Finally, based on the experimental results, a predictive formula for the contact stress of the gasket was established, considering the dislocation and opening amounts of the circumferential and longitudinal joints. The research results show that the contact stress is the highest at the intersection of the T-joint and decreases sharply within 20 mm from the intersection, gradually stabilizing beyond this region. The dislocation and opening amounts of the circumferential and longitudinal joints have a significant impact on the waterproof performance of the T-joint gasket, with the contact stress decreasing as the dislocation and opening amounts increase. The predictive formula proposed in this paper can estimate the waterproof performance of the gasket based on the dislocation and opening amounts of the segments.