Abstract: Study on how species coexists in a community is a central objective in ecology. Aiming to elucidate mechanisms that drive the observed pattern of fish species in the community within Sanmen Bay, fishes and six environmental influences (water depth, temperature, salinity, pH, dissolved oxygen and transparency) were sampled in September and November 2020. Redundancy analysis indicated that fish structure in the communities was related to transparency during September 2020, and water depth in November 2020. Null models showed that the observed Pianka indexes among fishes were statistically lower than simulated values in both months when the observed and simulated ones were 0.358 and 0.564 in September 2020 (P<0.01), and they were 0.477 and 0.546 during November 2020 (P<0.05), respectively, suggesting that the important role of competitive effects among species determining the structures of fish community within Sanmen Bay. Thus, our results reveal that fishery management strategies in Sanmen Bay should focus on promoting spatial and feeding niches relating to resources and habitat heterogeneities relating to habitat conditions in the system.