Abstract: Exploring the extent to which the construction and operation of the reservoir changes the hydrological regime of the river will help to deepen the understanding of the ecological impact of the reservoir and provide directions for the optimal operation of the reservoir. In this study,the measured hydrological data of the inbound station and the outbound station of the reservoir are proposed as the comparison object,with the help of index of hydrologic alternation and the range of variability approach proposed by Richer et al. to assess the influence of reservoir operation on river hydrological regime. Compared with the common method of using the measured hydrological data at the downstream hydrological stations before and after the completion of the reservoir as the comparison object,the suggested method can effectively avoid the disadvantages of confusing upstream human activities with the impact of the reservoir and a lack of data. Two representative reservoirs on the main stream of the Yellow River,Longyangxia and Xiaolangdi,are selected to calculate the influence of reservoir operation on the hydrological regime of the river. By the comparison of the calculation results with the commonly used method,the reliability of the calculation method proposed in this paper is confirmed. The results show that the operation of Longyangxia Reservoir has a great impact on runoff,the comprehensive change degree is 0.71. The indicators with small changes are mostly indicative of high flow,such as the monthly average flow in the flood season,the occurrence date of the annual maximum daily flow,and the number of high pulse occurrences per year,etc. The operation of Xiaolangdi Reservoir has moderate impacts on runoff and sediment,the comprehensive change degrees are 0.43 and 0.35,respectively.The impact on runoff is mainly reflected in the average flow in August,the annual minimum daily flow and the date of occurrence of the annual minimum daily flow. The impact on sediment is mainly concentrated in sediment content in the flood season,high sediment content events and changes in sediment contents. The research results can provide a certain reference for the optimal operation of the two reservoirs. It also points out the importance of constructing a multi-objective optimization model to find a scheduling plan that takes into account both reservoir operation goals and river ecological protection.