Abstract: By using the concept of moist potential vorticity,the article analyze a heavy rain in Guizhou on June 25th,2012,and discuss the relation ship between the heavy rain to the moist potential vorticity.The results show that the top of moist convection air can only attain between 600hPa and 500hPa levels,in the heavy rain.The tropopause folding,which means that high MPV spreads downward from the upper troposphere or stratosphere，is the main reason that cause the development of severe convectively weather.The MPV barotropic part is important,it reflect the releasing process of potential instability energy of severe convectively weather.The heavy rain region corresponds to the high MPV baroclinic part，and supplies an important reference for forecasting the intensity and the region of the heavy rain.