Abstract: The planktonic index of biotic integrity (P-IBI) has been extensively utilized to evaluate the ecological health of lakes. However, the key driving factors of the P-IBI during different hydrological periods are still unclear, particularly in seasonally ice-covered lakes. In this study, we examined the spatial and temporal pattern of the P-IBI index to evaluate the ecological health of the Xingkai Lake, China. A comprehensive P-IBI system was developed by incorporating the morphology-based functional groups into candidate metrics, and 12 indicators were determined for the P-IBI system construction, including different functional group traits. The comprehensive ecological health assessment of Xingkai Lake was ‘fair’ during all periods, with scores of the P-IBI ranging from ‘excellent’ (5.17) to ‘extremely poor’ (2.53). The ecological health showed significant spatial and temporal differences, with a relatively healthy status during the ice-covered period compared with other periods, and a relatively healthy status in the eastern region compared with the western region of Xingkai Lake. Phytoplankton Shannon-Wiener diversity index and Pielou evenness index were strongly related to the P-IBI index. Moreover, the key driving factors of ecological health differed during different periods, with dominant factors of common physicochemical parameters (salinity, NO₃-N, total nitrogen (TN), and total phosphorus (TP)) during the ice-covered period. Trace elements (Pb and Zn) played important roles in ecological health during the ice-freezing and ice-thawing periods. Our findings highlight the effect of trace elements on ecological health and aid in developing management strategies for eutrophic lakes.