Abstract: For realizing efficient utilization of water resource, reduction of surplus water, improvement of downstream navigation condition and more flexible operation of the power plant, the proprietor plans to install 3 Francis turbines with rated capacity of 480MW (rated water head 93m) in Xiangjiaba Hydropower Plant. On the basis of typical inflow series of different frequencies as well as daily average runoff series at Pingshan station, the additional power generation after capacity enlargement was calculated, and the comprehensive benefit after the enlargement was also analyzed. The research result shows that a certain amount of additional power generation would be produced by the enlargement. Under the influence of inflow re-distribution due to the newly-built reservoirs in upper reaches, the additional power generation would decrease, however the other comprehensive benefit would be significant due to the enlargement project.