By using the data from automatic ground stations, radar and NCEP reanalysis，the causes and thermodynamic conditions of a heavy rainfall event occurred in the centralsouthern Fujian on 19 April 2009 are analyzed. The results show that there were active mesoscale rain bands during this process; the mesoscale convergence zone at ground was a direct influencing factor; the southwest lowlevel jet provided water vapor for the occurrence and development of strong convection in the warm zone; the lowlevel warm air advection and highlevel cold air advection made unstable energy built up, and provided necessary energy for convection to develop.