Abstract: Human activities have great influence on runoff temporal change. The detected distinct transition point and relevant selected "unaffected" hydrological series that have great significance to assessment of soil and water conservation benefits. Taking Yanhe as an example,the distinct transition point of flood season runoff time series by the impacts of human activities has been detected by way of mathematical statistics analysis. And meanwhile,based on the datum before distinct transition point, the forecast model is set up only with the data series before the distinct transition point. The difference between the value derived from the equation and the observed value is right the influence on flood season runoff induced by human activities. Different from traditional methods,using hydrological datum and the synthesis coefficient of precipitation and runoff of flood season instead of annual ones,which could rationally weaken the impact of characteristics on rainfall and runoff in Loess Plateau. The analysis shows that 1970 is the transition point of runoff time series by impacts of human activities from the end of 1950s to 1980s,and human activities have affected significantly on hydrology of Yanhe basin since 1970.And in the year of 1981,human influence on river runoff reached maximum,with its value being 72.04%. In the 1970s and 1980s,flood season runoff was confined to reduced state at all times. So human factors is the primary one for the evolvement of Yanhe runoff.