The measured data of annual precipitation and runoff during 49years（1955-2003）for the small watershed of Zhifanggou located near Pingliang City,Gansu Province,were statistically analyzed in order to understand and quantify the contributions of human activities,and precipitation change to the variation of runoff on the Loess Plateau.It was found that although annual precipitation did not change significantly,annual runoff showed a tendency of significant reduction from 59.1mm in the 1950′s to 12.2mm in the early of the 21th century,a relative reduction of 72.4%.Based on the double-mass-curve of accumulated annual precipitation and annual runoff,the variation of precipitation-runoff was divided into five stages:1955-1964,1965-1976,1977-1985,1986-1995and 1996-2003.Then the stage of 1955-1964was chosen as the base stage for comparisons,and the contributions of human activities and precipitation change to annual runoff variation were separated and quantified for each stage.Besides the obvious contribution of annual precipitation decrease to annual runoff reduction in dry stage,the variations of annual runoff in other stages were mainly caused by the human activities,and the absolute contribution of human activities showed an increasing tendency.All these indicate that a rational planning of human activities is an important approach to solve the water problems on the Loess Plateau