Based on the daily Sea Surface Temperature (SST) data obtained by the modern high resolution radiometer of the National Oceanic and Atmospheric Administration (NOAA) during 1993 to 2012, with the methods of Season-reliant Empirical Orthogonal Function (S-EOF) and correlation analysis, the impact of SST anomaly of the Bay of Bengal on the onset of the South China Sea (SCS) summer monsoon was investigated. It is found that the Sea Surface Temperature Anomaly(SSTA) of the Bay of Bengal (6°-14°N, 85°-95°E)plays an important role in the onset of the SCS summer monsoon. A significant correlation passing 0.05 confidence level exists between the SSTA in the key area and the onset dates of the SCS summer monsoon, which shows that when the SSTs in the Bay of Bengal is higher (lower) than the normal, the SCS summer monsoon would occur later (earlier). Numerical experiments based on the atmospheric general circulation model ECHAM5 indicate that when SST in May decreases (increases) by 0.2℃ in the key area of Bay of Bengal, the SCS summer monsoon would occur earlier (later) about 5(10) days than the normal onset date, because the SSTA enhances (weakens) the cross-equatorial flows at 80°-100°E and then increases (decreases) the westerly over the SCS.