Focusing on the sharp decline of the runoff of Fenhe River into the Yellow River, based on the runoff data from 1960 to 2013 of the watershed controlled by the Fenhe reservoir, we analyzed the variation trend of the runoff by using the Mann-Kendall method and sliding T-test to look for the abrupt change points. We also used the double mass curve to calculate the influence of rainfall and human activities on the runoff decline. The results showed that the annual runoffs at Jingle station and Shangjingyou station both showed a trend of decline. The runoff series at Jingle station altered abruptly in 1979, and at Shangjingyou station, it altered abruptly in 1982. The runoff decline in the watershed controlled by Fenhe reservoir was mainly affected by human activities, whose contribution rate was 84.48%. The contribution rates of rainfall and human activities to the runoff at Jingle station were 15.62% and 84.38%, respectively, and those at Shangjingyou station were 14.95% and 85.05%, respectively.