Abstract: It is shown that the summer monsoon activity is not only associated with the movement of polar front and the west Pacific subtropical high but also closely related to the 100mb Tibetan high.The east-west or the north-south oscillation of Tibetan high will also influence the drought or floods of China.So that the above three factors are selected as the indexes to determine the date of onset and retreat of summer monsoon during recent thirty years(1951—1979). It is also found that the process of onset and retreat of summer monsoon has three basic patterns.The stepwise advancing pattern,the north-south oscillation pattern and the slow or stagnant pattern.But these patterns are always interalternated and their interannual variation is quite different.