Abstract: In this paper, the period of oscillation of the summer monsoon at low layer and precipitation of several stations in Yunnan are analysed by means of power-spectrum method from May to Oct. during 1985—1987. In the meantime the relationship of correlation and phase between the summer monsoon and the precipitation in apparent period are studied by using cross sdectrum method. It is found that the summer monsoon in Yunnan oscillates at similar periods with seasonal rainfall, especially well-correlated in periods that are significant. When the former is stranger than usual, there would be more rain. In contrast, when cold air occurs less frequenthy in weaker intensity in association with weaker summer monsoon, serious drought would take place in the year.