Few systematic investigations use isotopic compositions (delta O-18 and delta D) of seawater together with water temperature and salinity to trace the water dynamic processes in the upwelling area. This study presents depth-specific delta O-18, delta D, salinity (S), and temperature data of water samples collected from the northwestern South China Sea (NWSCS) during spring and summer 2018 to study upwelling and other oceanic physical processes. Strong upwelling is observed in the southwestern area of the NWSCS in both spring and summer based on the distribution of water temperature. In spring, the area with lower water temperature has generally higher delta D and delta O-18 values than other studied areas, while this phenomenon is not observed in summer. This indicates different hydrodynamic conditions in these seasons. The eastward advection of upwelled water in summer contributes to this phenomenon, which can be supported by the delta O-18-S relationships. delta O-18-S regression line in summer has higher slope than that in spring. Five water masses are defined in both seasons. In the west area of the NWSCS, the offshore upper water (OUW) is mainly formed by the mixing of coastal water and offshore deep water (ODW). We use the principle of mass conservation to estimate the relative contributions of ODW based on delta O-18. ODW in spring contributes 40% to OUW, while in summer it contributes 69%. This study presents the method of using stable isotopes to study the upwelling event in NWSCS and quantifies the contribution of upwelling which obtains more information