Abstract: Unsteady Reynolds-averaged Navier-Stokes equations of viscous uncompressible fluid have been solved for simulations of three-dimensional unsteady turbulent flows to study the capsizing stability of a ring gate of hydro-turbines during emergency shutdown. The fluid dynamics software FLUENT was used to calculate the capsizing moments acting on the ring gate during emergency shutdown. A mathematical model of the capsizing stability was developed on the basis of an analysis on gravitational effects and capsizing moments, and the capsizing stability was evaluated using the model parameters of a specific ring gate. The results show that this ring gate had certain capsizing risk under an unbalanced lateral force in the whole emergency shutdown process and its resultant capsizing moment reached the maximum at a closing stroke of 95%. During its entire dynamic closing from the start to complete closure, its lateral displacements were always within certain limits inside the allowed gap between it and stay vanes. Therefore, the ring gate will not get stuck on the stay vanes, maintaining a well running stability during emergency shutdown.