Abstract: Yuehai，a national wetland park, serves as a typical drainage lake in the Yellow River irrigation area in the upper reaches. Its water trophic state is a critical factor influencing the stability of its ecosystem. To investigate the evolution trends and main driving factors of the trophic state in Yuehai , this study analyzed long-term sequential water quality data from 2020 to 2024 using Mann-Kendall test，principal component analysis，and structural equation modeling to examine temporal changes and driving factors.The results indicated that the annual average of the comprehensive trophic level index for Yuehai ranged between 47.29 and 50.18 from 2020 to 2024, indicating a mesotrophic state overall with a risk of eutrophication. The monthly TLI(Σ) showed a significant increasing trend (Z=3.406, P=0.001), while no significant trend was observed on an annual scale (P = 0.083).The peak period of TLI(Σ) exhibited a delayed and sustained high-level pattern, with the key period for the lake's trophic state occurring from June to November each year. PCA revealed that the trophic state of Yuehai is primarily influenced by the synergistic effect of phosphorus and organic pollution combined with temperature on algal proliferation, supplemented by nitrogen input from ecological water replenishment and the coupling of phosphorus with physicochemical factors. TLI(Σ) showed significant correlations with key factors such as chlorophyll-a, transparency, and total phosphorus (P<0.01). The structural equation modeling results demonstrated that physicochemical factors, including pH, SD,and T,had a path coefficient of 0.367 with TLI(Σ), while the path coefficient for TP was 0.242. Both are identified as direct drivers collectively influencing the trophic state changes in Yuehai. This study can provide a reference for water environment quality management and algal bloom early warning and control in similar lakes.