Abstract: The seasonal and inter-annual variability of the South China Sea warm pool and its relations to the South China Sea monsoon onset were analyzed using Levitus and NCEP/NCAR OISST data. The results show that, there are obvious seasonal variability of the South China Sea warm pool, very weak in winter, rapid development in spring, strong and large in summer and earlier autumn, quick decay from mid-autumn. the thickness of South China Sea warm pool reaches 55 m in its strong period, its main axis is from southwest to northeast and locates alone the steep slope area of west off shore of North Kalimantan Palawan island; the South China Sea, Western Pacific and Indian Ocean warm pools have about 5 years period oscillation which is the same as ENSO cycle, the South China Sea and Indian Ocean warm pools have about 2.5 years period oscillation and similar inter-annual variation, and the South China Sea warm pool has best correlation with 5 months lag to the Western Pacific warm pool. The results also show that there are close relations of the South China Sea warm pool strength index to the South China Sea monsoon onset, when the South China Sea warm pool is continuously stronger (weaker) than normal in previous winter and spring, and the South China Sea monsoon will begin later (earlier) than the normal date, and this relation is connected with the activity of the South China Sea High in early summer.