Coastal wetlands (salt marshes, mangroves, seagrass beds) are critical hotspots in the global blue carbon cycle, among which mangrove ecosystems exhibit active carbon turnover and play a key regulatory role in coastal carbon cycling. Submarine groundwater discharge (SGD) has been recognized as an important hidden pathway for transporting terrestrial dissolved carbon to the ocean. However, its relationship with surface river discharge and the associated carbon effects remain unclear. This study was conducted in a mangrove system in Beihai, Guangxi. SGD fluxes under different river discharge conditions were quantified using a mass balance model based on four radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra, ²²⁸Ra). Simultaneously, the concentrations of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), dissolved carbon dioxide (CO₂), and methane (CH₄) in both groundwater and surface water were measured, and the carbon isotopic compositions of CO₂ and CH₄ were analyzed. Preliminary results show that under high, moderate, and low river discharge, SGD rates were 6.8 cm/d, 7.3 cm/d, and 9.1 cm/d, respectively, slightly higher than the previously reported 5.8 cm/d, and exhibiting a negative correlation with river discharge. The DIC and DOC fluxes delivered by SGD accounted for the vast majority of the total dissolved carbon input in the study area, further highlighting the dominant role of SGD in the nearshore carbon budget. Meanwhile, high concentrations of CO₂ and CH₄ were observed in groundwater, and preliminary carbon isotope tracing indicates that SGD input is a major driver of greenhouse gases in coastal waters. This study confirms the vital role of SGD in the carbon cycling and greenhouse gas emissions of mangrove ecosystems, and provides a multi-isotopic perspective for interpreting coastal carbon processes under river-groundwater interactions.