Abstract: In this paper, the torrential rain occurring over Yangtze River Basin in 20-23July 1998 is analyzed. Based on circulation and weather events corresponding to large-scale precipitation and meso-microscale torrential rain, convective instability and conditional symmetry instability in atmosphere are analyzed, the criteria of symmetry instability as well as vortex sheet instability are colculated and analyzed. Analyses of the convective instability and conditional symmetry instability indicate that the convective instability provides instable energy for the torrential rain, and energy of conditional symmetry instability also accumulates at 925-1000 hPa during the rainfalls. Because the criterion of the conditional symmetry instability is not satisfied above 700 hPa for all rainfall regions, it is not enough to explain the formation of the torrential rain if the conditional symmetry instability it self is considered. The torrential rain did not always occur in the areas where the criterion of convective instability and conditional symmetry instability are satisfied. On the condition of convective inst ability and conditional symmetry instability, it is easy to explain the torrential rain over Yangtze River Basin with the theory of vortex sheet instability. Calculation of the prerequisite of vortex sheet instability also indicates that vortex sheet instability is an important factor contributing to the torrential rain over the Yangtze River Basin in 20-23 July 1998.