Abstract: With the vortex motion stability method, combined the large-scale environmental field and the convective latent heat release of cumulus cloud, the physical mechanism of the development and extinction of the primary southwest vortex are studied. The results show that: (1) In the stable stratification and unstable stratification, when the atmospheric disturbance frequency exceeds the corresponding critical frequency, the primary southwest vortex can maintain and transform to the mature vortex. When the atmospheric disturbance frequency does not exceed the critical frequency, the primary southwest vortex is either perished by dispersion or perished after maintaining a pure vortex structure for a period of time, which cannot develop into a mature southwest vortex. (2) In the primary stage, the convergence and divergence of the large-scale field is the main factor for the development and extinction of the southwest vortex, and only the convergence can make the primary southwest vortex develop, while the divergence can only lead to the extinction. The small-scale latent heat heating determines the perturbation critical frequency under the stable stratification, and affects the development of the primary southwest vortex and the transition to the mature southwest vortex under the stable stratum.