Abstract: The effects of atmosphere general circulation background field and weather climate mechanism on the precipitation in May in Yunnan are studied. The results show that the precipitation in May of Yunnan is regulated by the seasonal variation of atmospheric circulation, the seasonal shift of cloud system from ITCZ in March to May, and by the locations of South Asia high and subtropical high. And it is close related with the interaction between the summer monsoon and the westerly trough, and with the interaction between the cold and warm air.