Based on meteorological observational data and mesoscale numerical model products, the physical mechanism of a heavy rain is analyzed. The analysis shows that sustaining meridional circulation at upper levels, low level warm shear line and jets, the strengthening of Southwest vortex and the obstruction from the high pressure on Japanese sea are the important reasons of the heavy rain. The main mechanisms are long-time transfer of vapor and energy by various jet streams, and the coupling among the strong vorticity column, violent convergence ascending motion and saturated air column.