For understanding to what extent the relationship between climatic factors and surface runoff been distorted by human activities, this study investigated the relationships between climatic factors, human activities and natural and measured runoff in the Weihe River drainage basin. The results show that the average annual natural runoff of the river during 1988-2008 decreased by 45% from that during 1956 1987, of which 33.0%, 15.9% and 51.1% were contributed by precipitation, temperature and human activities, respectively. Human activities played a more important role in this change. Nevertheless, the climatic factors can explain about 80% of deviation of measured runoff, which means the variation of measure runoff is highly consistent with climate change. The main reason for this is that the impact mechanisms of human activities and climatic factors on local runoff are different. Human activities, mainly water diversion, change the runoff by consuming the annual water resource in quantity. Although this may represent 50% or higher of the annual natural runoff, it can hardly change the long-term runoff trend, because water diversion itself is also partly controlled by climate. Climatic factors not only affect annual runoff quantity, but also control the long-term runoff variation, even if a large amount of natural runoff was consumed by humans. Therefore, the regression relationship between runoff and climatic factors can hardly be affected by human activities.