In the present project, niches and species association of seasonal dominant phytoplankton species in Dinghai Moat in Zhoushan City were analyzed, and then interspecific relationship of dominate species and seasonal changes of sharing environmental resources were discussed. A total of 14 dominant species of 12 genera were identified in the Dinghai Moat during four seasons from May 2016 to Feb. 2017, and there were six eutrophication indicator speices, Ulnaria acus, Aulacoseira granulata, Kamptonema chlorinum, Microcystis aeruginosa, Scenedesmus quadricauda and Cryptomonas rostrata. The dominant species changed obviously with the seasons, but Aphanocapsa elachista was dominated absolutely except in summer. According to dominance index and niche width, the 14 dominant species could be divided into three groups. The maximum and minimum niche overlap values were 0.984 and 0.004, respectively, and there was obvious difference among four seasons, winter ≈ autumn > summer > spring. The VR obtained by the variance ratio method were greater than 1 in each season, and their statistic W values indicated significantly overall positive correlation among dominant species. Based on the 2×2 linkage list and χ² test, only 11 pairs among 54 species pairs showed significant association and negatively associated species pairs were more than positively associated ones. As a whole, associated extent between dominant species was not in accordance with overlapped extent of niche, which is possibly resulted from high diversity of species and obvious seasonality of the species composition.