Abstract: Ensuring and managing the ecological flow of rivers and lakes is an essential requirement for strengthening the management and control of the development and utilization of water resources, and is an important part of the implementation of "Great Protection of the Yangtze River" and "high-quality development". A method for calculating ecological flow of rivers without measured runoff was proposed, with the Rangdu River taken as an example. Firstly, the monthly runoff process from 1990 to 2019 was simulated by SWAT hydrological model. The results showed that the runoff gradually increased from upstream to downstream, and presented obvious characteristics of flood season and non-flood season. Secondly, the ecological flow of the key control sections in the mainstream was calculated by four hydrological methods, e.g. Tennant method, minimum monthly average flow method, driest monthly average flow method in recent 10 years, and flow-duration curve method. The calculated ecological flow ranged from 10% to 33%, with the calculated flow trend in the order of flow-duration curve method > driest monthly average flow method in recent 10 years > minimum monthly average flow method >Tennant method. Finally, through water quality compliance accounting, it was concluded that the recommended method for calculating the ecological flow in the river was the flow-duration curve method, accounting for about 30% of the average annual flow.