Abstract: The Cladophora overgrowth has become a widespread global problem. The decomposition of Cladophora biomass releases large amounts of dissolved organic matter (DOM), a key substance in the biogeochemical cycle of carbon in aquatic ecosystems, resulting in a more complex organic pollution in water bodies. Field investigations and indoor experiments have proven that one of the sources of endogenous DOM in water bodies is the biodegradation of algae. Microorganisms play an important role in the decay of aquatic plants, and bacteria differ in their metabolic preferences and affinity for substrates. Bacterial diversity and community composition may both influence and respond to changes in DOM. However, it is not clear how the microbial community and the DOM composition of the surrounding water column change during the decomposition of the Cladophora. In order to study the composition of algal-derived DOM, we simulated the decay process of Cladophora in the laboratory. The experiment was carried out in a constant temperature incubator. 10 g (fresh weight) of Cladophora was placed into sterile polyethylene plastic bottles with 500 mL of sterile water and placed in dark conditions at 25℃ to decay naturally. Three replicate samples were randomly selected for chemical and microbiological analysis at 1, 4, 7, 10, 15, 20, 30 and 40 days. We performed 16S amplicon sequencing of algal-attached microorganisms to analyse the dynamic process of microbial self-assembly on decaying algae. The results showed that during the 40-day decomposition experiment, the biomass of Cladophora decreased, and it showed a trend of rapid loss in the early stages and slowed down in the later stages. At the end of the experiment (40 days), the dry matter residual rate was 43.15% and the mass loss was 56.85%. During the decomposition process of Cladophora, DOM quickly released to the maximum within 7—10 days. The composition of DOM also became complicated, and the fluorescence peaks gradually shifted from regions Ⅰ, Ⅱ and Ⅳ to regions Ⅲ and Ⅴ. A large amount of simple aromatic proteins, such as tyrosine, were transformed into various metabolites by microorganisms, and humic substances were produced. The dominant phyla of microorganisms attached to the Cladophora were Proteobacteria, Bacteroidetes and Firmicutes, with relative abundances ranging from 6.54% to 71.62%, 16.83% to 55.50% and 0.95% to 20.91%, respectively. In different stages of the decay process, the composition of microorganisms was significantly different, which was mainly dominated by Proteobacteria in the early stage and Bacteroidetes in the late stage of the experiment. Pearson's correlation (R=0.81, P=0.001) between Bray-Curtis distance and Euclidean distance of DOM composition for bacterial communities was calculated using the Mantel test (999 ranking). The results of the Mantel test indicated that changes in DOM composition were significantly correlated with changes in the composition of the bacterial community. These findings have implications for further understanding of the characteristics of DOM released during the decay of Cladophora blooms and the relationship between DOM and microbial communities, and provide theoretical support for the management of filamentous green algae blooms.