Abstract: The construction and operation of reservoirs have changed the riverine hydrological regimes, causing adverse effect on riverine ecological health. Previous studies focus on ecological operation of reservoirs in large-basin plains, while few researches are about mountainous medium-small basins. To explore the relationships between ecological flow and water resources utilization of reservoirs in mountainous medium-small basins, this paper selects the two medium and a large reservoirs in the Majinxi Basin as a case study. A multi-objective ecological operation of reservoirs model is established to minimize amended annual proportional flow deviation（AAPFD） of the Wentu Station and total water deficit of the water region while maximizing the hydropower generation. The non-dominated sorting genetic algorithm（NSGA-Ⅱ） is applied to solve this model to research the relationship among the objectives of ecology, hydro-electricity and water supply under conditions of different inflow frequencies such as P=50%, 75% and 95%. The results show:（1） with the increase in the inflow of the Majinxi Basin, the AAPFD of the Wentu Hydrological Station decreases, meanwhile the hydropower generation of reservoirs increases and the total water deficit of the water region declines, which improves the objectives of ecology, hydroelectric and water supply.（2） There is a competitive relationship among the objectives of ecology, power generation and water supply and the ecology objective has a stronger competitive relationship with the power generation objective than the water supply objective；（3） the typical non-inferior solutions with the optimal objectives of ecology, hydroelectricity and water supply are selected as schemes 1～3, respectively, and there is large variability in schemes 1 and 2 of multiobjectives while the scheme 3 is similar to a compromise scheme compared with the schemes 1 and 2. The research results can provide a reference for the ecological operation decision-making of reservoirs in the Majinxi Basin.