Abstract: This study examined how the bacterial communities on the surface of microplastics varied between winter and summer in Maowei sea, a typical subtropical bay heavily affected by high-density aquaculture in China. The main bacterial groups on the microplastic particles were Proteobacteria (30.52%～74.98%), Bacteroidetes (10.14%～58.64%), and Actinobacteria (1.21%～5.16%). Some pathogenic genera were detected on the microplastics, such as Vibrio (0.21%～3.26%), Pseudomonas (0.02%～2.88%), Acinetobacter (0.07%～0.95%), and Streptococcus (0.09%～0.86%). The bacterial species on the microplastics and in the seawater overlapped by more than 75%, but they differed in their community characteristics. The microplastics had lower bacterial species richness than the seawater. However, in areas with low salinity, the microplastics had significantly higher species richness than in other areas of higher salinity. The species richness and evenness of the bacterial communities on the microplastics were generally higher in summer than in winter. The results showed significant differences in the bacterial communities between microplastics and seawater and among different salinity regions in the Maowei sea. Water in aquaculture areas is a vital source for surface-attached bacterial communities on microplastics, where temperature and salinity are key factors shaping the structural characteristics. Notably, some pathogenic genera were detected on the microplastics.