The characteristics of interdecadal variability of precipitation in April and May and the associated atmospheric circulation, as well as the relationship with mid-West Pacific Ocean sea surface temperature (SST) on decadal scale over South China are analyzed by using observational station data, NOAA SST data, and NCEP reanalysis data for the period 1951-2005. The results show that both the April and May precipitation over South China exhibit a notable decadal change in the early 1970s, which experienced a notable low to high transition. The variability of April precipitation has a significant negative correlation with the Mid-West Pacific Ocean SST from July to November in the preceding year, and precipitation in May has a remarkable negative correlation with SST in the Mid-West Pacific Ocean from February to May in the same year. In the decadal period of lower (higher) rainfall in April and May, the Mid-West Pacific Ocean SST is warmer (colder) in the previous time period; the sea level pressure is significantly lower (higher) in southern Asian continent and the African continent at the same period, and the North Pacific Ocean sea level pressure is higher (lower); there appears an anomalous anticyclonic (cyclonic) circulation over South China, accompanied by a significant southerly (northerly) wind anomalies in the northern boundary of South China, causing a strengthened (weakened) of the anomalous water vapor output in the northern boundary of South China. Meanwhile, the middle and upper troposphere air temperature in the mainland of China is significantly warmer (colder), the East Asian jet stream is obviously northward (southward), the western Pacific subtropical high is weaker (stronger) and eastward (westward), so the water vapor transport to South China is weakened (enhanced), which emerges an anomalous divergence (convergence) of water vapor over South China, resulting in a decrease ( an increase) of April and May precipitation over South China.