Abstract: This study examines typical rainstorms on the western fringes of the Sichuan Basin from 2015 to 2018. The environmental field conditions associated with these rainstorms and their spatial distribution characteristics were also analyzed. Furthermore, it investigates the reasons behind the frequent occurrence of nighttime rainfall in the Sichuan Basin based on the atmospheric diurnal variation characteristics during summer. The results are as following: (1) The high and steep terrain of the Sichuan Basin’s western margin affects air temperature and humidity in the middle and lower layers of the western basin. This area is characterized by warm and humid southeast airflow that converges and lifts upon encountering large terrain, resulting in heavy rainfall. When rainstorms occur across the basin’s western region (referred to as west type), the humidity in the entire western basin is exceptionally high, accompanied by a strong southeast wind. When these storms occur in the basin’s northwestern region (the northwest type), the southeast wind at the middle and low levels is at its strongest, creating the most significant dynamic effect. However, rainstorms in the basin’s southwestern region (the southwest type) are usually accompanied by northerly wind entering the basin, resulting in noticeable north–south wind shear convergence. The water vapor divergence flux serves as a good indicator of the intensity and location of heavy rain in the western Sichuan Basin. (2) The diurnal variations of several factors, including the convective available potential energy, relative humidity, pseudo-equivalent potential temperature, and wind field at low and middle levels, indicate the tendency for precipitation to occur at night in the western Sichuan Basin.