Using NCEP global 1°×1° finalanalysis data and the conventional highaltit ude and surface data，the formation mechanism of the continuous rainstorm in the northwest side of subtropical high is discussed. The results show that the he avy rain happened in the background of the exceptionally powerful subtropical h igh and the strong Typhoon “Hagupit” landing westward, and there is no lowva lue system impact on 500hPa during the course. The main trigger systems are the subtropical high and continued strong southeast wind of lowlevel jet out of the typhoon. Continuous southeast wind of lowlevel jet provides rainstorm area the continuous water vapor and instability energy. The largest rapid zonal convergence speed and terrain elevation for the formation of rainstorm provide a driving force for increasing convergence conditions. The heavy rain district is located in a stable branch of ascendant circulation. In addition, the cold air in the proc ess played an important role.