The runoff characteristics in the Yongding River basin have changed enormously since the 1970s with part of the lower reaches drying up frequently all the year around. Based on the meteorological and hydrological data from two periods (1956-1991 and 2006-2018), the main influencing factors of runoff attenuation were analyzed using the M-K test and wavelet analysis. The attribution of runoff attenuation was performed using Double Mass Curve and Elastic Coefficient Method based on the Budyko hypothesis. The results illustrate: (1) during the years 1956-2018, rainfall in the Yongding River basin did not vary significantly, the temperature increased significantly, the evaporation capacity decreased slightly, and the runoff decreased significantly, reaching 99.9% confidence level; (2) the rainfall and runoff for most of stations have a fluctuation period between 11 and 23 years; (3) the double mass curves for 10 hydrological stations have shifted 4 times (mid-1960s, early 1970s, mid-1980s, and the early 1990s) which are all biased towards runoff. This indicates that the runoff has declined in spite of no obvious change in the rainfall level; (4) the Yongding River basin is a region affected by a lot of human activities. It should be noted that the absolute value of elasticity coefficient of precipitation is larger than the underlying surface parameters which in turn are larger than the evaporation capacity. Finally, the average contribution rate of human activities to runoff attenuation is about 95%.