Abstract: Using observational data, NCEP FNL analysis data and numerical model analysis data, the development characteristics, weather system conditions, ambient field factors and convection trigger of heavy rainfall in the south and west of Sichuan Basin are analyzed. The results show that there are three source regions of heavy rainfall in the west area of Sichuan Basin, which are the east slope of E’Mei Mountain, the south mountain region of Sichuan Basin and the Longmen Mountain in northwest of Sichuan Basin. The generation, development and extinction of heavy rainfall in the south and west of Sichuan Basin mainly present as developed type and undeveloped type. The weather system at 500 hPa level is the key factor to lead the development of heavy rainfall weather process in the south and west of the Basin under the same background in middle and low level of energy and moisture vapor field. Under the condition of insignificant circulation difference, the weak change of wind speed at 850 hPa level causes great difference on rainfall’s development. The bigger the wind speed at 850 hPa is, the better development of the heavy rainfall will be. In the analysis of ambient field, the most prominent factor is the water vapor. The content of ambient water vapor is not only the water vapor condition of deep wet convection, but also plays an important role in the unstable energy condition. Although the water vapor content in the Basin is abundant enough to trigger heavy precipitation process, the difference of water vapor content is still the key factor for the development of convective precipitation to upgrade status. The difference of water vapor content leads to the development trend of heavy rainfall. In turn, the development of heavy rainfall also affects the water vapor content threshold, and the two feed back to each other. In the analysis of heavy rainfall triggering conditions in the south and west of the Basin, attention should be paid to small-scale cold pool, topographic convergence and ground convergence lines. Moreover, the triggering configuration model of heavy rainfall convection in the south and west of the Basin is formed as low-pressure disturbance type and topographic convergence type.