Abstract: The diversion tunnel of Jinping Hydropower Station II is about 17 km long, and the flow inertia time constant is very large. With a huge installation capacity (8×600 MW), the hydropower station is very important in power grid. In construction, the arrangement of diversion system and the corresponding upstream and downstream  boundary conditions were adjusted due to the complex geological condition; to get a better hydraulic condition, a large and complicated differential surge chamber was arranged at the upstream. The small fluctuation stability was calculated and analyzed and the instability problem of isolated grid under extreme condition was studied. Based on the debugging result and test data of unit in field, some suggestions for the operation of the unit are provided.