Based on the conventional surface and intensive AWS rainfall data, satellite cloud images, and NCEP FNL global analysis datasets, the rainfall event over Hainan in autumn 2008 is studied. The results show that, interaction among the tropical depression, cold surge, and subtropical high induces the persistent low level easterly/southeasterly jet, which is an important trigger mechanism for MCS by the formation of on shore convergence and topographic lifting. The warm moist air transported by the low level jet is the main reason for the repeated development of convective instability. The strong low level convergence ahead of cold surge, latent heat release, and upper level divergence are benefitial to the developing and maintaining of tropical depression. The interaction between tropical depression and cold surge increases the baroclinicity, which would reinforce the low level northeasterly jet by quasi geostrophic and inertial oscillation processes. The strong convergence and deformation between the northeasterly and southeasterly jets result in the persistent frontogenesis. The ascending branch of direct thermal circulation forced by frontogenesis and the secondary circulation associated with low level jet are the main dynamic mechanism for the development of convective systems. And strong latent heat at low levels also leads convective systems to develop more vigorously.