Abstract:Based on the relative analysis of runoff of the Hongshui River and Sea Surface Temperature (SST) of Indian Ocean, the Eastern Tropical Indian Ocean Index (ETIOI) was established. Research shows that the index has a significant, persistent and stable inverse correlation with the runoff of Tianyi hydropower station in the Hongshui River. As the ETIOI was in cold phase, the South Asia monsoon and East Asia monsoon during the summer and autumn were stronger than usual, which triggers the southwesterly and easterly anomalies in the East Asia region, transports more water vapor from the Indian Ocean, the bay of Bengal, and northwestern Pacific to the eastern and southern China, and makes more cloud, stronger convection, and heavier rainfall, so the runoff of Tianyi became higher. However, as the ETIOI was in warm phase, the central, southern and southwestern China were controlled under the northerly anomalies, and dryer and cool air mass, so the water vapor supply declined, the convergence weakened, and the precipitation decreased, so the runoff of Tianyi became small. Hence, the key sea area SST index of tropical eastern Indian Ocean was as a factor to predict the runoff of Hongshui River.