Compared to most terrestrial forest ecosystems, mangrove forests have the higher carbon reserves as well as higher carbon emissions potential when they exposed to pollution. Mangroves have suffered heavy metal pollution to varying degrees in many regions and countries so far. Wetland sediments are an important part of mangrove ecosystems. The changes of total organic carbon in wetland sediments have a significant impact on the carbon sequestration capacity of mangrove ecosystem. Few studies have focused on the effects of heavy metals on the total organic carbon content (TOC) of sediments in mangrove wetlands compared to the assessment of heavy metal pollution in mangrove wetlands. The samples in this study were collected by 4 times from 0-30 cm of surface soil sediment in a typical mangrove wetland in the Pearl River Estuary during 2018-2019. The heavy metal content and TOC content in the samples were measured to explore the effect of the change in heavy metal content on TOC. The results showed that the study area has suffered heavy metal pollution, and the average value of heavy metals far exceeded the background value of soil elements in Guangdong Province. Through the principal component analysis (PCA) and related analysis, heavy metal pollution was confirmed to be mainly derived from human activities such as industrial production, mineral development, pesticide use, automobile exhaust emissions and aquaculture. The content of heavy metals in the mangrove wetlands would affect the carbon sequestration capacity of the mangrove wetlands and global warming because it could significantly affect the TOC content (P<0.01, R²=0.39). The environment with high Cd, As, and Zn content was favorable for the accumulation of TOC in the sediment, while the environment with high Cu, Cr, Ni, and Hg content had the opposite performance. The mechanism of the effects of heavy metals in sediments on TOC was very complex. The heavy metals in the sediment could cause changes in the TOC and carbon sequestration capacity of the sediment through a series of processes including affecting soil structure, soil chemical composition, soil microorganisms, growth of upper vegetation communities and litter return. This study analyzed the change of heavy metal element content in mangrove sediments and explored the relationship between heavy metal content and organic carbon content in the sediments, which plays an important role in assessing the environmental risks and carbon sequestration capacity of mangrove ecosystems. It will help to guide the ecological restoration and pollution control of the Guangdong-Hong Kong-Macao Greater Bay Area, promote the formation of green and low-carbon production and lifestyle and urban construction and operation models as well.