Abstract: Precipitation data from the National Climate Center, China Meteorological Administrition, NCEP/NCAR reanalysis data, and Sea Surface Temperature (SST) data from NOAA are used to study the climatological characters of vapor transportation route and the differences in transport mechanism for the month of May in abnormally rainy and rainless years in South China. The results show that, in rainy years, moisture comes from the South China Sea (SCS), the Bay of Bengal, and south of the Tibet Plateau. In rainless years the West Pacific Subtropical High (WPSH) is more eastern, and so vapor from the SCS, which is significant for the precipitation during May, cannot reach South China. When the SST anomalies in North Pacific have a quasi east-west “-＋-” pattern, anticyclones can be triggered in the SCS and WPSH can be enhanced and stretched westward. When the SST anomalies in North Pacific are “＋-＋”, cyclones can be triggered in the SCS, causing eastward weakening of the WPSH. The front of the southwest monsoon will be further south because of the barrier of cold air, and the intersection of the monsoon and cold air in South China will cause more precipitation anomalies.