Based on detailed geological survey and research of a underground watertight cavern, the stability of surrounding rock mass was evaluated by using the block theory. Vector analysis was used to analyze the movable blocks of each cut surface in the underground cavern, and mechanical analysis was employed to find out the key blocks of the movable blocks and to calculate their driving forces and failure modes. We conclude that this underground watertight cavern is stable, with the key blocks mainly sliding by single structural surface ② and ④, and mostly distributed on the roof of the cavern. Therefore, monitoring during the construction should be focused on places where the structural surface ② and ④ were exposed. At the same time, it’s found that the vector method could accurately evaluate the stability of surrounding rock blocks, but is needed to be improved when determining the specific location of key blocks and analyzing the stability of curved cavern walls.