Based on the observation, NCEP reanalysis data and the simulation of the mesoscale numerical model (WRF), the frontogenesis and conditional symmetric instability (CSI) during an extremely torrential rainfall event that occurred in southern China in 14-15 January 2012 is diagnosed. The results indicate that both the eastward propagation of the short wave trough along the southern branch trough and the cold air activity in the lower troposphere cause obvious frontogenesis in southern China, which is favorable for the torrential rainfall event in this region. The abnormal abundant water vapor transportation from the Bay of Bengal provides the moisture condition. The torrential rainfall is enhanced by the strong divergence at the right part of the exit of the abnormal strong higher level jet. The mesoscale rainbands form along the surface front. Analyses show the frontogenesis in Guangxi at the night of 14 January strengthens, thus the secondary frontogenesis circulation (SFC) develops. Affected by the latent heat releasing, the CSI is enhanced, leading to an intensifying upward movement in the warm side of the front. Under the circulation pattern, the strong mesoscale rainband occurs in the north part of Guangxi. Theoretic analyses reveal that both the SFC and the CSI are the solutions of the Sawyer Eliassen equation under different stable conditions, and the character of the SFC can be analyzed by the moist potential vorticity.