Abstract: Reservoir operation is an important driver to river flow regime alteration. Aiming at the problem of separating the influence of single factors of reservoir operation on flow regime from the compound influence of multiple factors,this paper proposes a way to quantify the influence of the reservoir operation on flow regime alteration. The daily runoff of downstream section of reservoirs with the absence of the reservoir is simulated. Indicators of Hydrologic Alteration（IHA）and Range of Variability Approach（RVA）is used to quantify flow regime alteration under the two scenarios of the presence or absence of the reservoir during the assessment period compared with the near-natural period.The influence of the reservoir on flow regime can be quantified by comparing the flow regime alteration differences. The influence of the Xiaolangdi Reservoir on the flow regime of Huayuankou section and Lijin section is evaluated. Results show that:compared with the near-natural period,the flow regime in the Lower Yellow River is altered sharply,featured by the decrease and flattening of runoff. Under irrigation,flood control,water and sediment regulation and other operating tasks,the Xiaolangdi Reservoir ensures no drying up in the Lower Yellow River continuously,increases the monthly average flow in January and March to July,decreases the magnitude of annual maxima 1-day means and3-day means,and increases the magnitude of annual maxima 1-day means,3-day means,7-day means,30-day means and 90-day means,and decreases the number of highly-changed and medium-changed IHA. Compared with the near-natural period,the engineering conditions,water consumption and natural runoff of the Yellow River has changed significantly from 2000 to 2019,which jointly led to the flow regime alteration in the Lower Yellow River. Therefore,the influence of the Xiaolangdi reservoir operation on flow regime is not completely consistent with the measured flow regime alteration. The results of this paper reveal that when the IHA – RVA is used to study the impact of reservoir operation on flow regime,it is necessary to eliminate the impact of climate change,water use change and other factors,so as to ensure the accuracy of the results.