Based on analysis and summary of the characteristics of ecological water demand of rivers, the lower envelope method based on the range of variability approach and results of multiple hydrological methods was proposed, and an evaluation system considering the accuracy of runoff data was established. A calculation framework of the ecological water demand of rivers considering the runoff inconsistency was constructed based on the consistency review and calculation and evaluation of the ecological water demand of rivers, and it was applied to the Hanjiang to Weihe River Water Diversion Project. The results show that the ecological water demand process of the river has the characteristics of volatility, seasonality, timeliness, and range; within the ecological water demand threshold interval, the lower envelope method can meet the requirements of maintaining river ecology and ensuring profitability, and it can further decrease the total ecological water demand with higher ecological water demand standards; the lower envelope method and the good and poor levels of the Tennant method can be used to obtain ideal, suitable, and minimum ecological water demand processes of the Hanjiang to Weihe River Water Diversion Project, respectively.