Based on the environment monitoring data from 1990 to 2020 and the aquatic plant survey data from 2011 to 2020 in Lake Honghu, the evolution mechanism and regime shift crucial threshold of Lake Honghu wetland are studied. The results showed that the evolution of water quality in Lake Honghu from 1990 to 2020 tended to be summarized into five stages and three periods, showing a deteriorating trend. From 2011 to 2020, the Margalef, Shannon-Wiener, Simpson and Pielou index of aquatic plants in Lake Honghu had decreased by 32%, 59%, 60% and 46% respectively, especially after the flood disaster in 2016. The crucial period for regime shift of Lake Honghu wetland ecosystem is 2002 and 2011 and total phosphorus is the key driving factor. The threshold value of total phosphorus from clean water to turbid water is 0.092 mg/L, and that from turbid water to clean water is 0.051 mg/L. This study provides a reference for the estimation of the pollution reduction load into Lake Honghu, and guidance for the priority control strategy of nutrients in the ecological restoration.