Using 1.0×1.0° NCEP reanalyzed data and based on the analysis of the synoptic situation of a rainstorm in late autumn, physical quantity diagnosis w as carried out. The results indicate that under the advantageous environmental b ackground situation, high potential vorticity extends from upper troposphere to low level and forms moist potential vorticity column, which causes the superposi tion of the cyclonic circulation and the low vortex circulation. The enhancement of moist baroclinity in the low troposphere causes the strengthening of the fro ntal zone in the low level and the development of vertical vorticity; the dry an d cold air invaded from upper level enters into the bottom layer, and the warm a nd wet air in the low level is forced to lift, which causes the surface cyclone to develop. The coupling of high and low level jet produces the upward curstronger compensation sinking movement excites the strengthening of lifting move ment, which produces the secondary circulation and triggers the release of unsta ble energy. The low and superlow level jets transport warm and wet air and ene r gy to Liaoning, so the moist air column is formed in the middle and low troposph ere and high unstable energy is accumulated. The appearance time, intensity, pos ition and structure of mesoscale cyclone, high and low level jets, moist poten tial vorticity column, the rising branch of secondary circulation, the surface we t area with high moisture content and high potential pseudoequivalent temperatu re decide occurring time and falling area of rainstorm.