Abstract: Ecological water demand is an important research direction in the fields of ecology, hydrology, environmental science, etc. The ecological restoration of rivers or the development and utilization of water resources require preliminary studies on the ecological water demand of rivers. How to scientifically determine river ecological water demand is a significant premise and basis for protecting river ecosystem and rationally allocating river water resources. At the present time, there are innumerable methods for calculating the ecological water demand of rivers. The wet perimeter method is a widely used hydraulic method for calculating the ecological water demand of rivers. The wet perimeter method is convenient and speedy to establish a reliable wet perimeter-flow relationship, but it is difficult to calculate the ecological water demand in ungauged watersheds. In order to solve this problem, this paper employs the aerial photogrammetry technology of Unmanned Aerial Vehicle（UAV） to obtain the large section shape of the river channel. The important hydraulic parameters, such as roughness and hydraulic slope, are acquired by analyzing and processing UAV images. Astable wet perimeter-discharge relationship is established with the Manning formula. The slope method is used to determine the curve inflection point, and the minimum ecological water demand of the river is calculated with reference to the inflection point. Two cross-sections（Huangta and Lianghekou） of the Nanhe River Basin in Qionglai City are selected as study sections to calculate the ecological flow. The conventional calculation approach with the Tennant method based on historical flow data is used as a comparison and reference to judge the rationality of the calculation results. The results indicate that the ecological flow of the two sections calculated by the wet perimeter method are 9.38 and 5.33 m~3/s, respectively, while the ecological flow calculated by Tennant method is 11.96 and 6.6 m~3/s. The two results are close and consistent with the flow distribution law, which proves that it is feasible to apply UAV technology to the wet cycle method to calculate river ecological water demand. The method for calculating river ecological water demand is feasible, providing a new way for calculating river ecological water demand in areas lacking data and an important basis for water resources development and utilization and water ecological protection. However, since UAV technology cannot measure large underwater sections, for the watersheds without hydrological observation stations nearby, how to carry out a reasonable cross section generalization and coupling with the above-water section when applying the wet cycle method, and how to verify the rationality of the calculated ecological flow and other issues need further research.