Abstract: The role of the western Pacific and the Indian Ocean in the South China Sea (SCS) summer monsoon onset is investigated by analyzing various atmospheric and oceanic datasets. The Results show that the impact factor of the SCS summer monsoon onset exhibits interdecadal variation. From 1951 to 1970, the impact factor exists in the Indian Ocean, from 1970 to 1998 it occurs in the western Pacific Ocean. The interdecadal variation of impact factor results from the significant abrupt change of the Arctic Oscillation (AO) and the intensity of the Western Pacific Subtropical High (WPSH). For the period of 1951 through 1970, an anomalous westerly wind over the northern Indian Ocean induced by a negative AO index and the weak WPSH that are helpful to the SCS summer monsoon onset make the zonal wind over the northern Indian Ocean the main factor controlling the SCS summer monsoon onset. Meanwhile, during the period, the southern Indian Ocean Subtropical Dipole (IOSD) is closely related to the SCS summer onset. The positive IOSD (with positive SST anomaly in the southwestern Indian Ocean, a negative SST anomaly in the other regions) results in anomalous westerly wind over the northern Indian Ocean and early onset of the SCS summer monsoon, and vice versa. For the period of 1971 to 1998, an anomalous easterly wind induced by a positive AO index and stronger WPSH offer an unhelpful condition for the SCS summer monsoon onset when the heat content in the western Pacific warm pool become the main factor controlling the SCS summer monsoon onset. The positive (negative) anomalous heat content in the western Pacific warm pool leads to earlier (later) onset of the SCS summer monsoon.