Mangrove forests, located at the interface of marine and terrestrial ecosystems in tropical and subtropical regions, harbor highly productive environments that support diverse microbial communities. These communities play a pivotal role in sustaining mangrove ecological functions and driving biogeochemical cycles. This study investigated the Ximen Island mangrove ecosystem in Zhejiang Province, employing high-throughput sequencing to systematically analyze the bacterial communities in surface sediments. We explored seasonal variations in bacterial diversity and assessed their correlations with environmental parameters. Results revealed that the sediment bacterial community in Ximen Island mangroves was dominated by Proteobacteria (39.11%), Desulfobacterota (10.02%), and Bacteroidota (9.17%). Bacterial diversity exhibited significant seasonal dynamics: Richness index peaked in winter (2136±235), while the Shannon index in spring (8.736±0.213) was significantly lower than in autumn and winter (P＜0.05). Principal component analysis (PCA) and Linear discriminant analysis effect size analyses (LefSe) results demonstrated seasonal specificity in bacterial community composition across spring, autumn and winter. Temperature and total carbon content were identified as key environmental drivers of these variations. By elucidating the seasonal succession patterns in microbial communities at Ximen Island, this study enhances the fundamental understanding of subtropical mangrove ecosystems at their northern distributional limit. Furthermore, it provides critical data for evaluating mangrove wetland restoration efficiency in Zhejiang Province and informs future conservation strategies.