Using conventional observation data, automatic station data, NCEP/NCAR (1°×1°) reanalysis data and WRF simulation data, this paper diagnostically analyzed the characteristics of the zonal rainband and its moist symmetric instability in the torrential rain that occurred in southern Chongqing and surrounding places from 27 to 28 June 2016. The results showed that: (1) The zonal rain belt was formed in the area with low-level moist symmetric instability. The moist symmetric instability increased before the formation of the zonal rain belt, and released gradually after the formation. The sustained release of moist symmetric instability promoted the continuous development of zonal rain belt. (2) The negative increase in horizontal component of moist potential vorticity MPV2 caused by the increase of zonal frontogenesis, was beneficial to the enhancement and maintenance of moist symmetric instability, and probably was the important mechanism of the formation and continuous development of the zonal rain belt in the torrential rain process.