Abstract: To study the effects of different modes of abalone mariculture on phytoplankton community structure and water environment, we carried out six surveys on the phytoplankton community and environmental factors in four mariculture areas (abalone culture area, Gracilaria lemaneiformis-abalone polyculture area, transition area and control area) in Shen′ao Bay, Nan′ao Island of Shantou City, from May to June, 2016. Altogether 84 species belonging to 45 genera of 4 phyla were recorded. The diatoms were the most dominant population with 37 genera and 71 species. For all culture areas, the average density of phytoplankton in after-harvest-period was higher than that in before-seaweed-harvest-period significantly (P < 0.05).The average density of phytoplankton in abalone culture area in before-harvest-period was higher than that in G.lemaneiformis-abalone polyculture area in after-harvest-period significantly (P < 0.05).In before-harvest-period, the concentrations of total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH₄⁺-N) and chlorophyll-a (Chl-a) in G.lemaneiformis-abalone polyculture area were the lowest, while the values of Shannon-Wiener index and Pielou′s were the highest. No significant difference was found in different culture areas in after-harvest-period. The phytoplankton community structure was affected by TN, TP, water temperature (T) and chlorophyll a significantly. It is indicated that G.lemaneiformis-ablone polyculture system can reduce the concentrations of nitrogen and phosphorus, maintaining relative stable community structure of phytoplankton.