Abstract: The operation of the Three Gorges Dam and the South-to-North Water Diversion Project has significantly changed the hydrological condition in the middle reaches of the Changjiang River.From 2020 to 2022, it showed the earlier and longer dry season in Honghu watershed.As a typical representative of shallow lakes in the middle reaches of the Changjiang River, the water environment of Honghu Lake has attracted considerable attention.In response to the challenges of insufficient water mobility and weakened self-purification capacity, Hubei Province plans to build a river-corridor to improve the regional water environment.The study explores the effect of large water-corridor in mitigating phosphorus pollution in plain lake regions under various water diversion scenarios with SWAT model, which provides scientific support for the ecological protection and management of Honghu watershed.The results showed that: ① The SWAT model had a good applicability effect in the plain lake regions of the middle reaches of the Changjiang River, the annual average phosphorus load was 744.90 tons from 2020 to 2022, and the results were close to those of numerous scientific research institutions, with a difference with in 6%.② The proportion of non-point source pollution was as high as 82%, which should be taken focus on the phosphorus pollution control in Honghu watershed.③ The construction of large scale river-corridor had a positive effect on improving the regional water eco-environment.Under the normalized water replenishment scenario, the cut rate of TP was 10.87%~21.66%, and the water exchange cycle of Honghu Lake decreased by 14.78%, which presented excellent level in the middle and lower reaches of the Changjiang River.