Abstract: Yuehai is a national wetland park and a typical drainage lake in the Yellow River upstream irrigation area. 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 employed Pearson correlation analysis, principal component analysis (PCA), and structural equation modeling (SEM) to analyze the temporal changes and driving factors, using five consecutive years of cross-section time-series water quality data from 2020 to 2024. The results showed that from 2020 to 2024, the annual average value of comprehensive trophic state index (TLI(Σ)) of Yuehai was between 47.14 and 50.29, and the trophic state was classified as mesotrophic to lightly eutrophic. On the annual scale, a declining trend in trophic state was observed. On the monthly scale, the trophic state was at a low level from January to March. Since April, the trophic state gradually increased with the increase of water temperature, farmland water withdrawal and ecological water supplement, and it was in a lightly eutrophic state from June to November. PCA and correlation analysis showed that Chla, SD and TP were significantly correlated with TLI (Σ) (p<0.01), which were the key factors affecting the trophic state of Yuehai. The SEM results showed that TP had a path coefficient of 0.242 to TLI(Σ) by promoting algae growth and increasing Chla levels, thereby affecting the trophic state. The path coefficient of physical factors (T, pH, SD) to TLI(Σ) was 0.367, which had a direct effect on trophic state, and regulated nutrients and Chla through indirect pathways. The unique hydrological characteristics of the closed lake in Yuehai result in its trophic state change being primarily influenced by phosphorus and physical factors. This study identifies the main controlling factors of the evolution process of the trophic state of Yuehai, providing a basis for the ecological protection and restoration of Yuehai.