Eutrophication is one of the main threats to aquatic ecosystems, and various nutrients combined flow into water. It is important to understand the responses of phytoplankton communities to enrichment of multiple nutrients for eutrophication control. To elucidate the response of phytoplankton communities to three essential nutrients (N, P and Fe) in tropical reservoirs of southern China, we analyzed the variation of N, P, Fe and Chl.a in a typical reservoir (Dashahe Reservoir) from dry seasaon (from January to March) to wet season (from April to June) 2021. An in-situ experiment was conducted to clarify the responses of phytoplankton abundance and diversity to the N, P and Fe enrichment. Compared with the conditions from January to March, the total phosphorus concentration increased significantly from April to June, the total Fe concentration showed an increasing trend but fluctuated greatly among months, total nitrogen concentration reached the lowest value in April then increased slowly. Accompanied with the variation of nutrients, Chl.a concentration increased from 5 μg/L in March to 29 μg/L in May. The results of in-situ enrichment experiment showed that cyanobacteria, chlorophyta and diatom were the main compositions of phytoplankton. The increase in phytoplankton abundance was observed in all treatments, but only the combined addition of both N and P (NP and FeNP) caused significant increase of biomass (indicated by Chl.a concentration). The abundance responses to the same nutrient addition were different among phyla and species, and community structure and dominant species were significant differences among treatments. The community diversity of treatments pronounced higher than control. Cyanobacteria abundance increased in all of the treatments (N, P, NP, FeN, FeP and FeNP) except the treatment of single Fe addition, chlorophyta abundance significantly increased in NP and FeNP treatments while no significant difference was observed between these two treatments, and abundance of diatom significantly increased in single addition of N and P treatments. These results suggested that phytoplankton abundance was primarily limited by N and P. Therefore, the increase in abundance depends on the synergistic rather than the single effect of nutrients. The any addition of N, P and Fe can affect community structure and diversity. Our study implied that eutrophication management, especially controlling cyanobacteria blooms, should consider the synergistic effect of N, P and Fe.