Abstract: Based on the chronological framework established by ²¹⁰Pb, total organic carbon (TOC) , total nitrogen (TN), stable carbon isotope (δ¹³C), and particle size indicators in sediment core S, this study reconstructed the development of mangrove forests in Trat Province over the past 100 years, and discussed the main influencing factors. Results showed that the TOC content in sediments ranged from 2.65% to 9.35%, gradually increasing from bottom to top, and the δ¹³C value ranged from −28.53‰ to −25.46‰, showing a negative shift trend from bottom to top. According to the end-member mixing model, the contribution of organic carbon sources is quantitatively calculated. The contribution of organic carbon in this region is mainly from terrestrial and mangrove sources, with relatively small contributions from marine sources. The development of mangrove forests in Trat Province over the past 100 years has been mainly influenced by air temperature, rainfall, human activities, and minimally regional hydrodynamics. From 1948 to 1962, human activities were relatively weak, and with the increase of air temperature and rainfall, the contribution of organic carbon from mangrove sources gradually increased. During the period 1962-2004, Marine aquaculture activities increased, and nutrient emissions from aquaculture stimulated the growth of mangrove forests. The deforested mangrove branches and leaves deposited in situ promoted the accumulation of mangrove-derived organic matter. Since the Indian Ocean tsunami in 2004, mangrove has played a prominent role in wave prevention and disaster reduction. The Thai government has performed large-scale mangrove planting. With the expansion of mangrove area, the contribution of mangrove-derived organic matter in sediment continues to increase.