Phytoplankton samples were collected both inside and outside a large scale raft mussel culture area of Gouqi island in September 2023. The diurnal variation characteristics of phytoplankton community inside and outside the culture area were analyzed by using abundance and diversity index and multivariate statistical methods. A total of 7 phyla and 158 species of phytoplankton were collected(103 species inside and 98 species outside the area), including 104 species of diatom(accounting for 65.82%), 30 species of Pyrrophyta(accounting for 18.99%), and 15.19% for the rest species. During the daytime, 94 species were collected in the inner area, while 103 species were collected at night. In the outer area, 93 species were collected during the daytime, and 98 species were collected at night. The dominant species of phytoplankton in the aquaculture area were 9 species, while in the outer area, there were 12 dominant species. Among them, Skeletonema costatum and Noctiluca scintillans were dominant species throughout the day in both areas. The average abundance of phytoplankton in the inner area was (1.77×10⁷±2.5×10⁶) cells/L during the daytime and (1.64×10⁷±2.31×10⁶) cells/L at night, while in the outer area, it was (1.87×10⁷±3.36×10⁶) cells/L during the daytime and (1.75×10⁷±2.72×10⁶) cells/L at night. The abundance variation in the inner area showed a multi-peak pattern, with peak values occurring during the rising and falling tides and the high tide. In contrast, the abundance variation in the outer area showed a single-peak pattern, closely related to tidal changes, with the peak value only occurring during the high tide.Non-metric multidimensional scaling (nMDS) analysis showed that there was no significant difference in phytoplankton community between day and night in the aquaculture area (P>0.05), but there was significant difference in phytoplankton community outside the area (P<0.05), which was significantly correlated with tidal changes (P<0.01). Temperature, dissolved oxygen and salinity were significantly correlated with the number and abundance of phytoplankton (P<0.05), the number of phytoplankton species was significantly negatively correlated with phosphate and nitrate (P<0.05), and the abundance was significantly positively correlated with ammonia nitrogen and silicate (P<0.05). The results showed that the impact of large-scale raft culture on phytoplankton community in local waters could present a more positive result at the stage of red tide organism aggregation.