Based on the typical rainstorm cases occurred in the western edge of Sichuan Basin from 2015 to 2018, the environmental field conditions of rainstorm with different spatial distribution characteristics were analyzed, and the reasons for frequent occurrence of night rain in Sichuan Basin were discussed from the perspective of atmospheric diurnal variation characteristics in summer. The results show that: (1) Influenced by the high and steep terrain in the western margin of Sichuan Basin, the air temperature and humidity in the middle and lower layers in the western Sichuan Basin are good, and the warm and humid southeast airflow converges and lifts when encountering the large terrain, which is prone to heavy rainfall. When the rainstorm occurs in the west of the whole basin (west type), the humidity in the west of the whole basin is very high and the southeast wind is strong; When the rainstorm occurs in the northwest of the basin (northwest type), the southeast wind in the middle and low levels is the strongest, and the dynamic effect is the most significant; When the rainstorm occurs in the southwest of the basin (southwest type), most of them are accompanied by northerly wind into the basin, and there is an obvious north-south wind shear convergence. The water vapor divergence flux is a good indicator for the intensity and location of heavy rain in the western Sichuan Basin. (2) The diurnal variations of the convective available potential energy, relative humidity, pseudo-equivalent potential temperature, wind field at low and middle levels and divergence all indicate that the precipitation in the western Sichuan Basin tends to occur at night.