【Objective】 The impacts of climate change and human activities on the decreased runoff in the Weihe Basin was studied to provide theoretical basis for use and management of water resources.【Method】 Accumulated variance analysis and Mann-Kendall test were used to identify the abrupt change point of runoff,and the method of comparing slope change ratio of cumulative quantity was applied to calculate the effects of climate change and human activities on runoff by taking the period before the abrupt change point as the base period and the period after the abrupt change point as the measurement period.【Result】 In Weihe Basin the abrupt change points of runoff were 1971 and 1991,respectively.During 1972 to 1991,the contribution of climate change was 14.60%,among which precipitation and evaporation were 24.55% and -9.96%,respectively.The contribution of human activities was 85.40%.During 1992 to 2005,the contribution of climate change was 30.37% (precipitation and evaporation were 25.00% and 5.37%,respectively) while the contribution of human activities was 69.63%.【Conclusion】 Human activities had the most important influence on decreasing trend of runoff with contribution of ~80% in Weihe Basin.