Abstract: River closure is not merely a key link in the construction of hydropower projects, but a control item that influences the entire construction progress. Along with the expansion of China’s hydropower development to the west, an increasing number of mountain river diversion and closure projects will be built. Based on a hydropower station project and using diversion and closure hydraulics along with model test data, this paper focuses on several key issues: immediate blocking and proceeding for protection of closure gap bottom and its effect, the effect of flow-deflecting secondary dyke on enhancement of the tunnel diversion ratio, the effect of rock pit at the diversion tunnel inlet on the diversion ratio, and whether or not to make further advance of the downstream cofferdam. Our research findings can solve these common problems in mountain river closure, thus ensuring a process of safe and reliable closure and remarkable economical benefits. This study would provide reference and guidance for closure design and construction of similar projects.