Abstract: Diagnostic analysis the first rainstorm of Xiluodu hydropower in early summer,2016. The results show: This process is due to the southwest vortex generator and remove the source. The water vapor transport under 700hPa is the source of the rainfall in Xiluodu. Positive vorticity and vertical upward movement area between 800 ~ 600hPa correspond better and they are the main driving force of the rainfall. The significant reduce of CAPE and there can be viewed as unstable rainfall precipitation transition to stability. When TBB value maintained at below -52℃,rainfall intensity is strong,but when TBB value increase rapidly,rainfall intensity is significantly reduce. The weak cold air intrusion of surface-layer can make the rainfall process more intense.