Abstract: Using conventional observation and EC numerical model data, the environmental conditions, water vapor conditions, energy conditions, unstable stratification and MCS system evolution of heavy rainstorm weather in Sichuan Basin from 12 to 13 August 2020 were analyzed. The results show that the heavy rain in the basin belongs to the ' two high shear ' type. Under the condition of high energy and high humidity, the northern dry and cold air invades the basin to trigger the front convective system, and the southern basin is affected by the forced uplift of the terrain to trigger the convective system, which is combined and strengthened on the upper shear line, resulting in the heavy rain process in Sichuan basin. The strong water vapor transport over the Guizhou Plateau is an important water vapor condition for this heavy rain, convective cloud cluster consolidation is an important feature of local torrential rain and the rainstorm area based on EC numerical model data analysis is clearer than the actual sounding.