We evaluated seasonal changes in the community structure of a demersal fishery in Laizhou Bay, China using data collected by beam trawl in May, August, October, and December of 2010. We captured fish at 19 sites in the study area (37°12′120°00′E). We collected a total of 78 species. The number of species was lower in spring than in other seasons. In terms of biomass, crustaceans were the most dominant category, followed by fish, cephalopods, and shellfish. The total biomass was highest in autumn, followed by spring, winter, and summer. The spatial distribution of biomass differed significantly among the seasons, primarily because of changes in the seasonal distribution of a few dominant species. We used the index of relative importance (IRI) to estimate the importance of each species in the community. The dominant species, which was defined based on an IRI value of >1000, were Leucosiidae spp., Diogenes spp., . The values for the Margalef richness index, Shannon-Wiener diversity index, and Pielou’s evenness were higher in summer than in the remaining seasons. We used Jaccard’s similarity index to assess the similarity of species among the seasons. Jaccard’s similarity was low between winter and the other seasons, but high between spring and summer, summer and autumn, and spring and autumn. We used ANOSIM to evaluate the dissimilarity of the fish community between seasons. The community similarity was low between spring and the other seasons both for biomass and abundance. We conclude that: (1) in Laizhou Bay, larger-sized, high-value, complex-age- structured species have been replaced by small, low-value, simple-age-structured species. (2) The mean body weight is low and the community is dominated by smaller individuals. (3) The community consists of relatively few dominant species, and is unstable, evidenced by seasonal replacement of the dominant species. (4) Beam trawling is harmful to the fishery and should be limited.