Mangrove fine roots are an important contributor to soil organic matter (SOC), yet how their production and decomposition affect SOC accumulation across the sea-land gradient is still elusive. This study aimed to investigate the influencing path of biotic and abiotic factors of fine root production and decomposition dynamics on SOC component ratios involving particulate organic carbon (POC)/SOC, dissolved organic carbon (DOC)/SOC, and microbial biomass carbon(MBC)/SOC. Based upon sequential root coring and the litter bag method, the production, and decomposition of mangrove fine roots were investigated. The production and turnover rate of mangrove fine roots increased from 90.9 g·m−2 to 510.9 g·m−2 and 0.44 yr−1 to 1.49 yr−1 across the sea-land gradient respectively. After one year of decomposition, 69.5–80.8% and 58.3–69.2% of low-order and high-order fine roots remained, respectively. Decomposition rates of both low-order and high-order fine roots were significantly slower in the landward zone than in the seaward zones. Redundancy analysis (RDA) illustrated that vegetation biomass and soil salinity were respectively the main biotic and abiotic factors affecting the production and decomposition of mangrove fine roots and they explained 91.2% of mangrove fine root production and decomposition. Stepwise regression analysis indicated that the production of fine roots was mainly influenced by mangrove vegetation biomass while their decomposition was negatively affected by soil salinity. POC and SOC increased significantly from 20.13 t·ha−1 to 58.71 t·ha−1 and 78.42 t·ha−1 to 139.01 t·ha−1 respectively while DOC/SOC and MBC/SOC reduced from 0.33 to 0.24 and from 0.55 to 0.44 across the sea-land gradient. RDA results indicated that low-order fine root production was the major control of the ratio of SOC components among the production and decomposition of low-order and high-order fine roots, explaining 90.9% of SOC component ratio changes. Based upon structural equation modeling, the path coefficient of biotic and abiotic factors POC/SOC was 0.914 (P < 0.001), indicating that mangrove fine root production affecting by vegetation biomass and soil salinity was the main drive of SOC accumulation. Nevertheless, mangrove fine root decomposition played a minor role in SOC component change. The information gained in this study will facilitate policy decisions concerning the restoration of mangrove forests in China.