Abstract: Seawater samples were collected from frequent harmful algal bloom areas in Haizhou Bay from March 2021 to January 2022. The diversity of cultural bacteria was evaluated by isolation, phenotypic identification, and 16S rDNA sequencing. A total of 641 bacterial strains belonging to 5 phyla, 7 classes, 79 genera, and 185 species were isolated. Among them, the phylum Pseudomonadota was dominant, followed by Firmicutes, Actinobacteria, Bacteroidetes, and Planctomycetota. The genera Pseudoalteromonas, Alteromonas, Pseudomonas, Sulfitobacter, and Qipengyuania were all detected in different batches and different water layer samples. Pseudoalteromonas was the predominant genus, suggesting that these genera are widely distributed cultivable heterotrophic bacteria in frequent harmful algal bloom areas. Diversity analysis reveal that the diversity indices of surface (S) and bottom (B) water layers were higher than that of the middle layer (M). The diversity index of culturable bacteria was higher in spring than in other seasons, reaching its peak in April. In addition, 18 strains with algicidal activity against Karlodinium veneficum were selected from 185 representative strains. This study provides scientific evidence for the microbial ecology of harmful algal bloom areas in coastal waters and provides bacterial resources for applications such as biological algal control.