Abstract: Based on the temperature and salinity from the Ssalto/Duacs altimetry data and the Hadely EN4.2.1 data, seasonal sea level variability and its underlying mechanism in the South China Sea are studied with the 1.5-layer reduced gravity model. The results show that seasonal sea level variability is significant and exhibits opposite spatial variation in winter and summer, spring and autumn. The highest sea level appears in November and the lowest value appears in June, the magnitude of sea level variability is about 10 cm. Seasonal variability signal can explain 70%−95% of the monthly mean sea level variance in northwestern and southwest shelves of the South China Sea, but can only explain about 20%−60% of variance in internal and eastern areas of the South China Sea. Steric effect is important to sea level variability in northeastern South China Sea, and can explaine about 50% of the seasonal variability, while wind stress forcing plays a leading role in central South China Sea, and explains about 50%−80% of the variance. The wind stress forcing that results in sea level change in central South China Sea comes from the remote forcing from Tropical Pacific and the local forcing in the South China Sea.