Abstract: This study investigated the phytoplankton community structure, dominant species' niche characteristics, and interspecific association in the Donghu Reservoir, Datun Reservoir, and Shuangwangcheng Reservoir along the Eastern Route of the South-to-North Water Diversion Project in Shandong Province. During the survey, 8 phyla, 59 genera, and 116species of phytoplankton were identified, with Cyanophyta, Chlorophyta, and Bacillariophyta being the dominant groups. The phytoplankton communities in the reservoirs exhibited distinct seasonal succession patterns, that is, spring was characterized by a co-dominance of Bacillariophyta and Chrysophyta; Cyanophyta was absolutely dominant in summer and autumn, with harmful bloom-forming cyanobacteria such as Microcystis spp, Pseudoanabaena limnetica, and Cylindrospermopsis raciborskii were the dominant species; while Cyanophyta and Bacillariophyta were co-dominant phylum, with abundance of Pseudoanabaena limnetica reaching as high as 7.17×10⁷ cells/L. Niche analysis revealed that the average niche width (B_i) of dominant species in Cyanophyta increased by 77.3% and 34.0% in summer and autumn, respectively, indicating significant expansion during these seasons. Seasonal changes in niche overlap (ΔSO_ij) showed that Chlorophyta and Bacillariophyta were in developmental stages in spring, while the niche overlap of Cyanophyta continued to rise to 77.8% in summer and autumn, but the resource competitiveness of Cyanophyta gradually declined in autumn. Interspecific association analysis revealed that there was an overall non-significant negative association in spring and summer, and a significant positive association between the Cyanophyta and Chlorophyta in autumn, reflecting the functional complementarity and niche differentiation among these species. Among the 924 dominant species pairs, 57.47% showed significant association, with as many as 506 pairs having extremely significant negative associations, indicating that the community as a whole was in a state of intense competition. Based on these findings, it is recommended to incorporate positively associated species pairs with high niche overlap as water bloom early warning indicators into the risk management system, providing a scientific basis for ensuring drinking water safety.