Based on the routine meteorological observations, Doppler radar data a nd the numerical simulation outputs, a mesoscale analysis of a torrential rain e vent over Liaodong Peninsula on 3 August 2004 is made. And a diagnosis of the ev ent is made by using the theory of moist potential vorticity. The results show t hat the mesoscale systems causing the torrential rain are a low level jet (LLJ), a shear line and a mesoscale depression. The meso-γ scale echoes, bandi nband echo, mesoscale cyclone, LLJ and bow echo detected by Doppler radar cause stron g convective rainfall. The analysis of simulation outputs indicates that LLJ and the two different upper level jetLLJ couplings made an important contribution to the torrential rain by producing strong ascending motion. The horizontal mois t isentropes turning to very stiff and dense state led to the development of moist slantwise vorticity and the amplification of the precipitation. The strong co nvective instability and the baroclinic instability in the low level provided th e conditions of the torrential rain. 