Abstract: Recently, Sanya Bay, Hainan Island, South China, suffered from beach blackening, which has become an environmental issue affecting local tourism and ecological health. After the field investigation, we conducted a geochemical analysis on grain size, organic matter content, and different iron forms of offshore and beach sediments in Sanya Bay. Results show that the surface sediments are mainly silty. The grain sizes become finer with less sandy and more silty and clayey contents from nearshore to offshore. This distribution pattern indicates that the hydrodynamic force in the bay is weak and it is enhanced in the nearshore. The main sources of organic matter of beach blackening in surface sediments of the Sanya Bay are the overflow of rainwater and sewage, the "sapropel" from the sediments of Sanya River, and the resuspension and redeposition of sediment loaded from the previously discharge of organic pollutants. Clay minerals are not the main reason for the enrichment of organic matter. SEM-EDS analysis shows that the main blackening minerals are iron oxides. Iron morphology analysis show that the complexities of iron oxides and free iron are positively correlated with organic carbon content. In the weakly alkaline oxidation environment, iron oxides adsorb organic matter via complexation by ligand exchange, and metal ions promote the adsorption of organic matter by cation bridging. Therefore, the interaction of iron oxides, organic matter, and heavy metals shall be responsible for the cementation of polluted organic matter and formation of the beach blackening in Sanya Bay.