Abstract: Using NCEP FNL 6 h reanalysis data with 1°×1° resolution and 6 h precipitation data at 124 stations in Yunnan, the atmospheric circulation characteristics and the instability conditions of a typical Sichuan-Yunnan shear rainstorm event are analyzed. The results indicate that the precipitation is produced under the influence of multi-scale system and terrain. The key factor for the precipitation is that the Sichuan-Yunnan shear stays in the mountain range combination of Yunling mountain-Ailao mountain for long periods. The mountain range combination of Yunling mountain-Ailao mountain serves as the dividing line, there are more precipitation in the southwest side and less precipitation in the northeast side. The result of moist potential vorticity (MPV) analysis shows that the distribution and change of instability conditions (convective instability and conditional symmetric instability) have a very good correspondence to distribution and change of rainstorm. In main precipitation period, the middle and lower troposphere in the strong precipitation area is in the state of the convective instability, while weak precipitation area is in the state of conditional symmetric instability. The formation, distribution and maintenance of instability conditions are related to the secondary circulation caused by the upper jet stream.