Yangshan Port is an international port with a complex water environment, located on an island more than 30 kilometers from mainland China. Human activities, such as port construction and shipping, have a great impact on the environment around the port. Researchers have investigated and analyzed the archaeal diversity in the surface seawater of Yangshan Port; however, the diversity of bacteria in Yangshan Port has not been studied in depth. Seasonal variation of bacterial abundance and characteristics of community composition in Yangshan Port were investigated by using a high-throughput sequencing approach. We explored the seasonal variation in dominant groups, total cultivable bacteria, and relationships between microbes and environmental parameters. Results indicated that the density of total cultivable bacteria in Yangshan Port was 1.2-9.1×10⁴ CFU/mL, which was lower than in other offshore areas and ports. The density of total cultivable bacteria was highest in summer, with a significant difference compared to the other three seasons; the order of total cultivable bacteria across various seasons was summer > autumn > winter > spring. Results also indicated that the dominant groups differed across seasons. Diversity analyses revealed the presence of Proteobacteria, Bacteroidetes, and Cyanobacteria. Dominant groups were γ-proteobacteria and α-proteobacteria form autumn to spring, while Flavobacteriales dominated in summer. Autumn and winter were dominated by the Halomonas and Pseudomonas genera that were replaced by Synechococcus and Flavobacteriaceae in summer. The proportion of Halomonas was approximately 50% in autumn, winter, and spring; Halomonas was the most competitive of the bacterial community groups, however, there were also seasonal differences in its growth. Compared to the other three seasons, microbes were more abundant and diverse in summer; the order of bacterial diversity across various seasons was summer > spring > winter > autumn. Bacterial community composition was relatively simple with low levels of diversity throughout autumn and winter. Canonical correspondence analysis was used to explore the relationship between microbes and environmental parameters of Yangshan Port. The most significant factors affecting bacterial dominance were total organic carbon, salinity, chlorophyll-a, and temperature. Seasonal changes in temperature, salinity, total organic carbon, and other important abiotic factors affected the bacterial biomass, community composition, and dominant groups of microorganisms in Yangshan Port.