The ecological flow is the key to grasping the ecological environment and water resources in rivers. To resolve the issue of being incapable of determining seasonal river ecological flow due to the large inter-annual and intra-annual variations in river hydrological processes, this paper proposes a Copula-based Flow Duration Curves (CFDC) method that takes into account the “cross-phenomenon” between the annual and monthly inflow types. The ecological flow process of the seasonal river, the Dawen River, was calculated based on the natural monthly streamflow data at the three hydrological stations from 1956 to 2016. The results indicated that the ecological flow variation ranges at Beiwang station, Loude station, and Daicun Dam station were as follows: during wet years, 2.96-102.79 m³/s, 0.60-70.39 m³/s, and 9.29-201.95 m³/s, respectively; during normal years, 2.56-47.68 m³/s, 0.45-23.40 m³/s, and 5.59-66.97 m³/s, respectively; and during dry years, 2.20-13.90 m³/s, 0.51-8.65 m³/s, and 4.49-38.17 m³/s, respectively. The ecological flow determined by the Copula-based Flow Duration Curves (CFDC) method fulfills the requirements of river ecological health and is scientific and reasonable. This achievement not only improves the applicability of the hydrological method to determine ecological flow, but also provides a theoretical basis for ecological restoration and comprehensive management of water resources in the Dawen River.