Abstract: This study aims to reveal the impact of the Indo-Pacific warm pool on the onset of the South China Sea summer monsoon. Based on the fifth generation of global reanalysis data from the European Center for Medium-Range Weather Forecasts, it is found that the warmest Sea Surface Temperature (SST) above 30℃ over the Indo-Pacific warm pool shifts during May. The warmest water appears in the central Bay of Bengal in early May, then disappears and shifts to the southern part of the South China Sea in late May. Analysis of local ocean-atmosphere coupling at the synoptic scale reveals the underlying mechanism for the shift. After the onset of the Bengal summer monsoon, the gradually strengthening latent heat release and reduced short-wave radiation lead to gradual reduction in the area of warmest water in the Bay of Bengal. Meanwhile, under the influence of the subtropical high, warmer water appears and expands over southwest of the Philippine Islands, which, together with the warm water at the Gulf of Thailand, lead to warm water in the southern South China Sea. It is found that the summer monsoon onset in the South China Sea often follows the warmest water shift in the Indo-Pacific warm pool. Therefore, the transition of the SST difference between the central Bay of Bengal and the southern South China Sea from positive to negative can serve as a precursor for the onset of the South China Sea monsoon.