The reasons for five severe rain events and unusually great rainfall in May 2001 in Yunnan area are mainly caused by transversal shear produced at the southeastern side of the Qinghai Xizang high on 500hPa, the warm wet southwest air current in the periphery of subtropical high and weakly surface cold air.Furthermore,Iran high moves eastward quickly to Qinghai Xizang Plateau impelling high circulation on 500hPa and 700hPa settled in the Plateau for a long time,cold air and plateau transversal shear affects Yunnan area conscantly,which are main reasons of unusually early rainy season and formation of severe rainfall.