The Ecopath model was used to study the food web and energy flow in the Manwan reservoir ecosystem in China. The study found that the ecosystem is still developing, with a dominant detritus pathway and a high energy flow from detritus. Piscivorous fishes were the top predators, with an ecotrophic efficiency of 0.073, while zoobenthos had an efficiency of 0.93. The ecosystem had a total system throughput of 5550 t km−2 yr−1, a primary production/respiration ratio of 2.46, and an ascendency index of 27.4%. These findings are important for preserving biodiversity and managing reservoir ecosystems for aquatic organisms.