Based on conventional observational data and mesoscale numerical simulation results, a rare heavy rainfall event occurred in the southern Shandong Province between 17 and 18 August 2009 is analyzed. The results indicate that the rainstorm was influenced jointly by the subtropical anticyclone, the upperlevel westerly trough, and the ground inverted trough. The strong south airflow set up the water vapor channel where water vapor can flow to Shandong Province continuously. Meanwhile，the convective unstable stratification was formed over Shandong Province. During the heavy rainstorm process, there was a strong mesoβvortex system over the rainfall area, which was characterized by strong and narrow vertical ascending motion, a vertical structure with convergence at the lower levels and divergence at upper levels, and strong convective instability.