Abstract: To explore the impact of Ulva prolifera green tide on the concentration and chemical properties of dissolved organic matter (DOM) in different bloom stages, three field surveys were carried out in the Southern Yellow Sea. The investigations focused on analyzing the DOM concentration and fluorescence composition characteristics in seawater before, during, and after the Ulva prolifera blooms. Results showed that the dissolved organic carbon (DOC) concentration reached the peak of (182.99 ± 35.85) µmol/L after the green tide, which was 69.73% and 51.66% higher than those during the pre-outbreak and outbreak periods, respectively. The concentration of dissolved organic nitrogen (DON) was the highest before the outbreak (13.26 ± 3.00) µmol/L. The DON concentrations during and after the outbreak decreased significantly by 44.72% and 29.11% respectively (P＜0.05) compared to that before the outbreak. According to main components of fluorescent dissolved organic matter (FDOM), the intensity of protein-like substances showed a significant increase during the outbreak process, which was usually used as an indicator of labile dissolved organic matter (LDOM). In contrast, the humus-like substance, which often represents recalcitrant dissolved organic matter (RDOM) component, showed a slight accumulation after the outbreak. This suggested that the majority of macroalgal biomass carbon is bioavailable and easily utilized by microorganisms. Moreover, due to the mineralization of LDOM by microorganisms, the macroalgal DOM after the outbreak consists mainly of recalcitrant components. Therefore, it is speculated that the macroalgal blooms can increase the concentration of LDOM in the coastal water in the short-term and have the potential to increase the accumulation of RDOM pool in the long-term. This study provides a novel insight into the characterization of changes in chemical components of DOM before, during and after the green tides in the Southern Yellow Sea.