Abstract: Runoff in many rivers in China have presented decreasing trend over the past decades. How to identify the effects of climate change and human activities in the trend variation are a current hot topic,a difficulty problem as well. An assessment method to quantitatively distinguish the effects of climate change and human activities was put forward in the paper. SIMHYD rainfall runoff model was briefly introduced and calibrated with "natural" hydro-meteorological data in Fenhe River basin. Based on simulation of natural runoff in human-disturbed period with hydrological model,causes of runoff variation were analyzed. And results show that SIMHYD rainfall runoff model has good performance for natural monthly discharge simulation. On average,35.9% and 64.1% of total annual runoff reduction from 1970-1999 were induced by climate change and human activities respectively. Human activities are main reasons of runoff reduction in Fenhe River basin.