The heavy rain process in the southern Guizhou Province during 24, 25 June 2007 was simulated successfully by using the threelevel nested mesoscale numerical model MM5 V3.5. By means of the simulated high resolution output data, the formation mechanisms of a heavy rain and the mesoscale low vortex were analyzed and diagnosed. It is shown that the model can successfully simulate the development of mesoscale systems and the distribution characteristics of precipitation. The development and maintenance of a mesoscale low vortex is the direct cause of the heavy rain. Heavy rainfall occurred in the southwest of a low vortex. A deep ascending motion column formed under the favorable condition: atmosphere positive vorticity, convergence, and strong ascending motion at lowerlevels, while atmosphere negative vorticity and divergence at upper levels. The mesoscale dynamical structure had a good relation to the occurrence time and location of heavy rainfall and was one of the dynamic mechanisms of maintaining and growing of the mesoscale low vortex. The area of heavy rainfall corresponded well to the strong ascending motion and the positive vortex areas.