Abstract: Based on the NCEP reanalysis data and ERSST sea surface temperature (SST) data, using statistical methods, 10-30-day significant sub-seasonal variability periods are extracted from the summer (Jun-Aug) convective in the tropical western Pacific, and these oscillations have different effects on the South China Sea summer monsoon intensity at different scales. At inter-annual time scale, a positive significant correlation is found between the intensity variation of 10-30-day oscillation over the tropical northwest Pacific Ocean regions (TWPI), and the correlation coefficient is 0.635. Influences of TWPI on the South China Sea summer monsoon intensity are mainly regulated by ENSO. Because of the asymmetric response of the lower troposphere Northwest Pacific atmospheric circulation to ENSO, TWPI is much more significant in El Niño developing years than in La Niña years. During strong TWPI years, the SST anomalies are El Niño pattern, which induces anomalously enhanced westerly in the South China Sea, the Philippines and the tropical northwest Pacific Ocean. The westerly anomalies generate strong positive vorticity shear, resulting in abnormal cyclonic circulation, and enhance TWPI and summer monsoon intensity through the wind-evaporation feedback mechanism. On the contrary, in La Niña years, the anticyclonic anomaly circulation result in TWPI weakening and the monsoon strength weakening. Under different inter-decadal backgrounds, TWPI does not show a significant change in the decadal trend, mainly slightly weaker (1958-1976), slightly stronger (1977-1993) and slightly weaker (1994-2011) change. The trend of the summer monsoon intensity inter-decadal changes are more obvious, namely, much stronger, slightly weaker and much weaker. The overall change of vertical shear and water vapor-convection are consistent with TWPI, but are not consistent with the summer monsoon. Vertical shear of wind field and water vapor-convection play important roles on the inter-decadal variation of TWPI, but not for the summer monsoon. The thermal contrast between sea and land is the key factor that leads to the inter-decadal change in the South China Sea summer monsoon.