Planktonic bacteria play a key role in biogeochemical cycling in marine environment. To investigate the community structure of planktonic bacteria across summer and winter and their key environmental drivers, the surface seawater samples at 0.5m depth from seven stations were collected in June and December, 2017, respectivly, in the Maowei Sea, Beibu Gulf, Guangxi. We then analyzed the community structure and diversity of planktonic bacteria using 16S rRNA gene high-throughput sequencing. Our results showed that the planktonic bacteria community in the Maowei Sea belonged to 11 phyla, 36 classes, 86 orders, 188 families, and 506 genera. Proteobacteria (52.47%), Firmicutes (17.34%), Actinobacteria (16.39%), Bacteroidetes (7.54%), and Cyanobacteria (5.38%) were the major phyla, which accounting for 99.12% of the total abundance of the planktonic bacteria. The Chao 1 Index and Richness Index of the planktonic bacterial community in summer samples were significantly higher than that in winter in the Maowei Sea of Beibu Gulf (P<0.05). Additionally, the results of PCoA analysis based on Bray-Curtis distance and ANOSIM (R=0.586, P<0.001) indicated that there were significant differences in the planktonic bacterial community structure between the two seasonal groups. The Welch's t-test indicated that the abundance of 15 orders significantly varied between summer and winter (P<0.05). Among them, Euzebyales, Rhodobacterales, SAR11, Flavobacteriales, Lactobacillales, Bacillales, and Pseudomonadales were significantly increased in summer group, Oceanospirillales, Propionibacteriales, Nevskiales, Vibrionales, Burkholderiales, Mycobacteriales, Rhodospirillales, and Nitrosomonadales were significantly increased in winter group. The co-occurrence network analysis based on SparCC indicated that a more complex topological feature of the bacterial co-occurrence network was found in summer than that in winter. Furthermore, the inter-species interactions of the co-occurrence network were mainly positive relationships, which were greater in summer than that in winter. And the proportion of positive correlation inter-species is 70.3% in summer group, and that is 68.9% in winter group. The partial Mantel test demonstrated that the planktonic bacterial community was significantly positively (P<0.05) correlated with temperature, salinity, dissolved oxygen, total dissolved nitrogen, and total dissolved phosphorus. Among these factors, temperature (R=0.326, P<0.001) and total dissolved phosphorus (R=0.512, P<0.001) with the highest correlation degree with the community structure of the planktonic bacteria in the Maowei Sea, Beibu Gulf, which were identified as the key factors that affecting the community structure of planktonic bacteria. Overall, this work provides a scientific basis for revealing marine planktonic bacteria diversity in Beibu Gulf, as well as solid evidences for ecological restoration and resource utilization in the Maowei Sea.