Abstract: The analysis in this paper based on the atmospheric observation data and the model data of higher resolution shows that one of the major synoptic systems that caused the flashfloodproducing rainstorm in mid June 2008 in Guangxi is a quasi-stationary mesoscale vortex. The vortex was resulted from the interaction of synoptic westerly waves in the mid-latitude with the warmmoist flow in the low-latitude under the terrain effect. The genesis, development and movement of the mesoscale vortex have important influence on the precipitation intensity and continuance in the process of the flash-flood-producing rainstorm. The vortex has the characteristics of strong vorticity and divergence coexisting, and the two have the same order of magnitude as each other. A long-life organized deep and moist convection is accompanied with the mesoscale vortex. Gravity waves, which are generated by terrain effects and ageostrophic effects of large wind speed center in a low-level jet, interact with the mesoscale vortex, and as a result, they stimulate a β-mesoscale organized deep and moist convection of large amplitude. According to the theory of quasi-balanced dynamics, quasi-balanced flow is suitable to describe such mesoscale motions containing both divergent wind and rotational wind. Thus, the PV-ω method, which is based on the analysis of quasi balanced flow, is introduced in this rainstorm process, and is applied in the inversion and diagnostic analysis of the organized deep and moist convection of long life with vorticity and divergence coexising. The results show that the quasi-balanced flow can be a true reflection of the characteristics of large-amplitude vertical motion in which vorticity and divergence coexist. 50%—70% of the vertical circulation in the rainstorm area belong to the quasi-balanced flow. Therefore, the quasi-balanced flow is able to describe a deep and moist convection with a long life organized course. The quasi-balanced flow field has the characteristics of coexistence of vorticity and divergence motion.