Abstract: In order to analyze the impact of Danjiangkou Reservoir operation on downstream hydrological regime, based on the mean daily flow data from 1961 to 2010 at Huangjiagang, Xiangyang and Huangzhuang stations, the indictors of hydrological alteration (IHA) and range of hydrological variation method (RVA) were used to analyze quantitatively the hydrological regimes and the hydrological alteration degrees in middle and lower reaches of Hanjiang River before and after the operation of Danjiangkou Reservoir. The results show that four of the five IHAs have changed significantly except for the timing of annual extreme flow that remained invariable. The overall degree of hydrological alteration of Huangjiagang, Xiangyang and Huangzhuang stations were at medium, low and medium level respectively. The alternation has an adverse impact on ecosystem stability and biodiversity in the middle and lower reaches of Hanjiang River. The analysis result will provide a scientific basis for ecological operation of Danjiangkou Reservoir and ecological restoration in the middle and lower reaches of Hanjiang River.