Remediating water eutrophication is critical for maintaining healthy and sustainable development of lakes. The aim of this study was to explore the seasonal variation in phosphorus (P) speciation and bacterial community structure in sediments of Qin Lake (Taizhou, Jiangsu Province, China) associated with the growth of submerged macrophyte Vallisneria natans. The differences in sediment bacterial diversity and community structure between V. natans growing and control areas were analyzed over a period of one year. The results showed that V. natans growth reduced the total P and organic matter contents of the sediments and increased the bioavailable iron (Fe) and Fe-bound P contents. The α-diversity of sediment bacteria was significantly higher in the presence of V. natans than in the controls during the vigorous plant growth stage. In the presence of V. natans, there was a higher relative abundance of Proteobacteria and lower relative abundances of Chloroflexi and Acidobacteria. The Fe(II) content in the sediment had a larger influence on the spatial distribution of bacterial communities than sediment Fe-bound P, organic matter, and Fe(II) contents. V. natans growth could reshape sediment bacterial community structure in the shallow lake, which, in turn, enhanced P immobilization in the sediments and thereby improved the water quality.