In this study we investigated the projections of climate change and its impacts on the water resources of the Xin’anjiang watershed and optimal hydropower production using future run-offs (the decades of the 2020s, 2050s, and 2080s). The arc SWAT hydrological model and change factor downscaling technique were integrated to detect the run-offs and to downscale CMIP5 future climate variables, respectively. Optimal hydropower generation using future runoff was predicted by developing a mathematical model and by applying the particle swarm optimization technique within its paradigm. The results depict an increase of up to 5.9ºC in monthly mean maximum temperature, and 5.58ºC in minimum temperature until the 2080s. There will be a 63% increase in flow magnitudes more than the base year flow during the 2020s, whereas up to 70% and 31.40% increments have been observed for the 2050s and 2080s, respectively. The results revealed potential hydropower generation of 19.23*10⁸ kWh using 2020s runoff of rainy years. Similarly, 19.35*10⁸ kWh and 14.23*10⁸ kWh were estimated from the flows during the 2050s and 2080s, respectively.