Using the conventional observed data, the NCEP 1°×1° grid point material, cloudimages and CINRADCC Doppler weather radar data, and the mesoscale numerical model WRF, an extremely heavy rainstorm occurring in the middle of Yunnan Province on 2 July 2008 has been diagnosed and simulated. The result shows that the heavy rain is caused by the dry intrusion from the upper troposphere，cold and warm advection confluence in the low middle layer and abundant vapors from the Bay of Bengal. In this extremely heavy rain process, the most effects of vertical helix are in the middle layer, supreme positive potential vorticity (PV) exists on the tropopause, the subsupreme positive PV is 〖HJ〗under 300 hPa, and between the two layers present columnar distributing characteristics. Meanwhile, the subsupreme positive PV can affect its lower layers. The simulated rain and reflectivity distributions by WRF mesoscale numerical model show that the characteristic of rain belt and generant time in Yunnan Province are well reproduced. The change trend of simulated maximum convective available potential energy (CAPE) can well indicate the rain occurrence. The simulation result is better within 30 hours.