The key environmental factors (nutrients, Chl a, COD, et al.) and eutrophication indexes was investigated in the surface water surrounding the Nan’ao Island in 2022. The seasonal change of nutrient concentration was significant, which gradually declined from northwest to southeast in the surrounding region of Nan’ao Island. The nutrients concentration was significantly higher in winter than other seasons. During summer, the nutrient concentration was high near the Qin’ao Bay, which possibly originated from the discharge of domestic wastewater. The highest dissolved inorganic nitrogen(DIN) was found near the estuary of Rongjiang River and the highest dissolved inorganic phosphorus(DIP) was found near the net cage aquaculture zone in the Zhelin Bay. This phenomenon suggested that the river input increased the nitrogen loading and the cage aquaculture increased the phosphorus loading in this region. The seawater showed high Chl a and low nutrient in summer, suggesting the strong bio-assimilation effect of phytoplankton on the nutrient. The structure of nutrients was generally balanced, and no absolute limitation was detected for nitrogen, phosphorus or silicate. Nitrogen nutrient was relatively limit in the offshore region in autumn. According to the Eutrophication index(EI), Nutrient quality index(NQI), and trophic index(TRIX), the highest eutrophic status was generally near the Zhelin Bay and Rongjiang River estuary. However, the region near Qing’ao Bay showed high eutrophic status in summer. With the fast development of tourism, the influence of domestic wastewater should be paid more attention in Nan’ao Island. The eutrophication indices showed a significantly positive correlation with DIN and a negative correlation with salinity. River discharge should be the main reason causing water eutrophication in this region. The eutrophic status might be underestimated using EI index in the waters with low-nutrient and high-chlorophyll during summer. The results contribute to a better understanding of the water quality status surrounding the Nan’ao Island and provide basic data for environment management and marine resource utilization.