The climate is complicated in Yunnan Province,China due to its special position in the lower latitude area,its heavy rainfall process is quite different from those in East and South China.Most of heavy rainfalls and floods occurred with the local characteristics.In May 2001,a record heavy rainfalls since 1949 appeared in Yunnan except its northeast part.During 3 days,from 1200 UTC 30 May to 1200 UTC 2 June,the strongest precipitation appeared in Yunnan province.In this paper,analysis has been done by using the conventional data,intensive precipitation data and hourly satellite data.The results show that the heavy rainfall was caused by mesoscale systems under the favorable large scale environment: 1) The interaction between Indian-Myanmar trough and East Asia colder trough made instability energy releasing and reappearance of convection.At the same time,the maintenance of Indian-Myanmar trough was very favorable to the water vapor transportation to heavy rainfall area.2)There may existed the certain close relationship between lower level jet and heavy rainfall.The lower level jet not only provided abundant water vapor,but also produce instability energy.3) Under the favorable large scale environment,the interaction between mesoscale convergence lines near the ground was probably the important triggering mechanism for the occurrence of heavy rainfall.And the convergence lines,closely relating to the terrain of Yunnan Province,were very different from that of South China.4) The special topography in Yunnan Province and its adjoining area also played the important role in the triggering of the heavy rainfalls.Most of rainy clusters produced and disappeared in same area,forming two maxima precipitationl centers and mesoscale convective cloud cluster related with mesoscale convergence lines closely.5) The analysis for the three-dimensional structure of precipitation systems shows,in the lower troposphere,mesoscale convective systems developed,the convergence was strong,the relative vorticity was positive;in the upper troposphere,the divergence stronger,the relative vorticity was negative.There existed a whole air column with strong upward motion,and downward motion in both sides of it.In this convective cloud column,there existed rich water vapor as well as instability energy.6) The analysis for water vapor budget indicates,the source of water vapor for heavy rainfall may be mainly in the Bay of Bengal.