The carbon cycle within the upper water column of the South China Sea exhibits distinct seasonal fluctuations, closely associated with upper ocean mixing driven by the monsoon. However, the response of stable carbon isotopes(δ13C) of surface ocean dissolved inorganic carbon to the variations in ocean mixing on the seasonal scale, remains poorly understood due to the limited observational data and reconstructions. Analysis of published planktic foraminifer Globigerinoides ruber δ13C records obtained from sediment traps in the southern South China Sea and reanalysis datasets reveals a direct correlation between the seasonal changes in δ13C of surface seawater and the upper ocean mixing driven by monsoon and buoyancy flux. In this study, we selected a sediment core MD05-2896 (08°49.05'N, 111°26.47'E; water depth of 1657 m), which drilled from the southern South China Sea in the 2005 cruise MD147, and focused on the upper 3.305 m of the core(total core length of 11.03 m). And then, the seasonal amplitude of dissolved inorganic carbon δ13C in surface seawater since the Last Glacial Maximum in the southern South China Sea was reconstructed using the standard deviation of individual planktonic foraminifera δ13C (δ13CIFA) measurements of G.ruber from core MD05-2896, and indicating weak variation on the seasonal amplitude of G.ruber δ13C. Combined with the paleoclimatical simulation results, it is hypothesized that the monsoon-driven Ekman transport and the buoyancy flux during the inter-monsoon period together effect the upper ocean mixing in the southern South China Sea, leading to the maintenance of a long-term stable trend in the seasonality of G.ruber δ13C.