Abstract: Lake regime shift from macrophyte-dominated to algae-dominated steady state under the background of water body eutrophication, is a common phenomenon of lake ecosystem degradation. Taking Lake Gehu as the research object, this paper analysed the time points, driving factors and their thresholds of ecosystem regime shift. The data used covered a period of 1986-2022 and included hydrometeorological data, water quality, macrophyte and algae characteristic indicators, and aquaculture area. There existed the multimodal phenomenon of the frequency of macrophyte and algae characteristics indicators at different levels of eutrophication. The obvious jumps were also evidenced in the relationships of macrophyte and algae characteristic indicators with typical environmental factors. These proved the existence of alternative stable states in the Lake Gehu ecosystem. Generalized additive model (GAM) revealed that total phosphorus (TP) concentration and transparency were the main driving factors for the changes in macrophyte and algae characteristic indicators in Lake Gehu. Based on the above data and the TP and transparency thresholds, the regime shift of Lake Gehu ecosystem could be divided into three phases: (1) From 1986 to 2000, the Lake was characterized as a stable state dominated by macrophytes. At this stage, TP concentration＜0.078 mg/L and the transparency ≥ 0.6 m; (2) From 2000 to 2007, the Lake was in a macrophyte-algae transitional state. At this stage, TP concentration ranged from 0.078 mg/L to 0.13 mg/L; (3) From 2007 to 2022, the Lake was in a stable state dominated by algae. At this stage, TP concentration＞0.13 mg/L and the transparency＜0.6 m. This study contributed to an improved understanding of the mechanisms of regime shift in Lake Gehu and provided early warning and guidance for lake management.