Coastal reservoirs receive and accumulate a large amount of carbon; therefore, they are important areas for global carbon cycling. The biogeochemical cycles of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) are important components of coastal reservior carbon cycle, with significant effects on biological process and environmental change. This study was conducted to analyze the spatiotemporal variation in DOC and DIC concentrations at Wenwusha Reservoir, a subtropical coastal reservoir in Southeast China, based on the high spatial resolution filed sampling in November 2018 (autumn), March 2019 (spring), and June 2019 (summer). The results showed that the DOC and DIC concentrations of the surface water in Wenwusha Reservoir during the research period changed in the ranges of 0.10-21.13 mg/L, 0.38-34.94 mg/L, with mean values of 4.09±0.18 mg/L and 15.83±0.24 mg/L, respectively. Temporally, concentrations of DOC and DIC in the reservoir showed the highest values in summer and autumn, respectively. Spatially, DOC and DIC concentrations in the northern part of the reservoir were higher than those in the southern part. Furthermore, the DOC and DIC concentrations decreased along a gradient from shallow water (<1 m) to deeper water (>3 m) in the reservoir. The seasonal variations in DOC and DIC concentrations were mainly influenced by phytoplankton photosynthesis, while the spatial changes of DOC and DIC concentrations were primarily affected by exogenous input (e.g., sewage discharge and river flow). The results improve the understanding of carbon cycling in aquatic ecosystem and also be helpful for improving water quality in reservoirs.