Utilizing the NCEP/NCAR reanalysis data, the characteristics of annual south wind over East Asia are investigated. And it is discovered that a key area (22.5°-30°N,105°-110°E) where the south wind prevails all the year is over the southeast of the Tibetan Plateau. The south wind is strong in summer and weak in winter, showing a double peak. A peak occurs in the 15th pentad and the other peak occurs in the 37th pentad. Further research found that there is a good relationship between the south wind on the southeast of Tibetan Plateau and the spring precipitation in southern China. In the 15th pentad, the south wind on the southeast of Tibetan Plateau becomes stronger and expands eastwardly, and then the rainfall in the south of China becomes heavy. The occurring reason for the two peaks of the south wind is different. The south wind peak occurring in the 15th pentad is for a low level cyclone caused by the Tibetan Plateau heating superimposed over the south wind. It causes the south wind stronger, and then South China comes into rainy period. The south wind peak occurring in the 37th pentad is while the South China Sea monsoon broke out, the southwest wind in front of the Bay of Bengal trough intensifies the south wind. And then it brings about the rain over the Yangtze River.