By use of conventional data, as well as AWS, FY2C TBB and Doppler weather radar data, the causes of a regional rainstorm occurred in Shandong from 9 to 10 May 2009 are analyzed. The results indicate: (1) The severe precipitation was produced when the cold air met with the southwest warm wet air. The stronger subtropical high made the surface convergence line and the shear line at 850 and 700 hPa stagnated; therefore, the severe precipitation is maintained persistently. (2) The formation and maintenance of the surface convergence lines stimulated ascending motion in the boundary layer and provided sufficient water vapor for the storm, and the cold air met with the warm wet air, leading to the convergence and updraft. As a result, the ascending movement strengthened and the precipitation increased. (3) The mesoscale convective system (MCS) was the major influencing system, and many multicell storms alternated and moved through the same region to form “train effect” and produce a regional rainstorm. (4) The lowlevel jets of different heights and the head wind zone in the radar radial velocity chart have indicative significance to severe precipitation forecasting.