A heavy rainfall event induced by the moving and developing southwest vortex occurred in the middlelower reaches of the Yangtze River from 6 to 7 June 2013. Using conventional observation and NCEP reanalysis data, diagnostic analysis was done on this event, especially on the physical mechanism of the vortex moving and developing and on the variability of the rainfall climax. The results show that the southwest vortex moving easterly along the 700 hPa shear line is the direct impact system to the heavy rainfall. Southwest vortex moves eastward and develops, and when reaching its deep phase, positive vorticity column stretches up to 400 hPa height with a nearly vertical structure in the relative vorticity chart. Largescale condition of lowerlevel convergence and upperlevel divergence in the vicinity of southwest vortex, dynamic structure of the coupling of southwest vortex and southwest lowlevel jet, positive vorticity advection in front of the highlevel trough and other favorable weather conditions are the main causes for the vortex reinforcement when it gets to the middlelower reaches of the Yangtze River. Heavy rainfall area that accompanies the southwest vortex is mainly located within 3 latitudes to the south and east side of the vortex. Convergence of two water vapor advection provides abundant humidity and latent unstable energy for the rainfall. One is the southwest monsoon flow, and the other is the southeast flow from the southwest side of the tropical high. Besides, cold air from the middlelower level of troposphere coming down to the rear side of the vortex reinforces the development of vortex, promoting an unstable stratification, which provides convective triggering mechanism to the heavy precipitation.