Abstract: Dissolved organic matter (DOM) plays a vital role in the carbon cycling process of coastal wetlands. Seasonal changes in hydrological process in the coastal area will affect DOM composition, thus controlling nitrogen transport and transformation pathways. In this study, the coastal area of Lianyungang, Jiangsu Province, was selected as the study area to investigate the seasonal characteristics of DOM and its impacts on nitrogen transformations based on the hydrochemical data of groundwater, river water, and seawater, combined with three-dimensional fluorescence spectroscopy and UV-visible spectroscopy. The results show that DOM in coastal area includes three components, terrestrial source-like fulvic acid (C1), terrestrial source-like humic acid (C2) and microbial source-like protein fraction (C3). In the study area, the NH₄-N and DOC concentrations gradually increased with closer proximity to the coast, the N concentration is associated to the DOM Component characteristics. During the wet season, the groundwater was recharged by rainfall and river water, with exogenous macromolecules DOM infiltrated accompanied by NH₄-N into groundwater. The aquifer is in biased reducing condition, and the nitrification process is inhibited. During the dry season, the interaction between salty and fresh water was weak, at when the higher C3 component in the groundwater. Meanwhile, the aquifer is in more oxidizing environment, promoting nitrification. In the intertidal groundwaters, the DOM is characterized by higher degree of humification, the abundance of NH₄-N and DOC suggests the mineralization of N contained soil organic matter. Also, a longer retention time of water as well as a strong microbial activity is likely to promote the dissimilatory nitrate reduction to ammonium (DNRA) and lead to further accumulation of NH₄-N.