Using mesoscale simulation WRF model,the formation and development of a southwest vortex occurred over Sichuan province on June 30 and July 1,2008 were investigated.The results indicated that the southwest vortex primarily emerged at 850 hPa and then formed at 700 hPa after several hours.The formation of the southwest vortex at 850 hPa was directly connected with low-level jet.The diagnostic analysis of ω equation showed that the horizontal advection of vorticity and divergence mainly influenced the formation of the southwest vortex at 850 hPa while latent heat release was not important to its genesis.The results also indicated that the latent heating was the key factor for the formation of the southwest vortex at 700 hPa.The dry sensitive experiment further confirmed that latent heat release had little influence on the southwest vortex at 850 hPa,but it caused strong convergence of cyclonic shear circulation and finally formed the vortex at 700 hPa.