As the key part of offshore drilling drawworks brake system, the brake disc plays a vital role in guaranteeing the working reliability and operational security of the drawworks. To obtain the distributions and variations of thermal stress field in the water-cooling bake disc in an emergency braking, the 3D thermo-mechanical coupling theoretical model and FEM were established in this paper. Meanwhile the displacement and thermal boundary conditions for solution were determined, and then fully coupled analysis of thermal stress field in the disc was carried out by using ABAQUS software. The analysis results showed that, temperature field and stress field in the process of emergency braking were fully coupled. The temperature, radial stress and circumferential stress on the disc surface were presented as a hackle. The circumferential stress was significantly greater than the radial stress. Thermal stress has a periodic effect on the brake disc during braking, so the circumferential stress is the main factor that accounts for the initiation and propagation of crack on the brake disc surface.