Abstract: Dissolved organic matter (DOM) in coastal groundwater is derived from multiple sources including marine water, river and leachate from sediments. Quantitative estimation of the contribution of difference sources to coastal groundwater is importing for understanding the carbon reactive transport in the coastal area. Isotopic tracer, fluorescent indices, and end-element mixing analysis (EMMA) were used to identify the sources of DOM in coastal groundwater and quantify their contributions. The results showed that DOM in coastal groundwater was mainly originated from DOM in river water, seawater and sediments, with contributions of 44.0% ± 22.2%、33.0% ± 10.8% and 22.9% ± 13.1%, respectively. Groundwater collected from the north part of study is affected by both seawater intrusion and fresh water recharge, showing a high proportion of seawater DOM and autogenous characteristics. In the south part, groundwater has higher total dissolved solids (TDS), saline water releases DOM in sediments, showing higher proportion of DOM in sediments and humification characteristics. This study demonstrates the DOM in coastal groundwater is affected by hydrodynamic and hydrogeological conditions. End-element mixing analysis based on fluorescent indices can efficiently quantify the sources of DOM in coastal groundwater, which provides a new perspective and assessment method for the study of terrestrial-oceanic carbon cycle.