Abstract: The daily daytime/nighttime precipitation data of 81 National Meteorological Science Data Center stations on the Qinghai–Tibet Plateau and 80 stations in Southwest China were used to statistically analyze the daytime/nighttime rainy days of different magnitudes, differences in this number in the two regions from 1961 to 2020, and interannual change (trend) characteristics. The results showed the following: (1) The number of daytime/nighttime rainy days of different magnitudes on the Tibetan Plateau and in Southwest China increased rapidly in May and decreased significantly in November; large differences in the number of daytime/nighttime rainy days in the two regions were observed in May–October, and the number of heavy rain and rainstorm days was higher in May–October. The number of rainstorm days on the Tibetan Plateau accounted for ＞85% of the total number of rainstorm days in the year, heavy rain accounted for ＞90%, and the proportions of these two levels of precipitation were 93% and 86%, respectively, in Southwest China. The probability of extremely heavy precipitation (rainstorms) during the rainy season in both regions was substantially higher at night than during the day. (2) In addition to obvious regional differences in the number of daytime/nighttime rainy days of different magnitudes in the two regions during the rainy season, very significant differences were observed between daytime and nighttime. The Three-Rivers Source region and the southeastern part of the Tibetan Plateau have frequent light and moderate rains during the day and night. Heavy rain occurs exclusively at night in southeastern Tibet and the east of the Three-Rivers Source region; rainstorms occur only in the east of the Tibetan Plateau, and there are visibly more sites receiving precipitation at night than during the day. Central Sichuan, western Sichuan marginal mountains, and southwestern Yunnan in Southwest China have a high incidence of daytime/nighttime light rain and moderate rain; heavy rain is more likely to occur during the day in southwestern Yunnan, whereas it is more likely to occur at night in central Sichuan. Typical areas with rainstorms are central and eastern Sichuan, western Chongqing, and central and southern Guizhou. (3) The overall trends of interannual changes in the number of rainy seasons in the rainy season are significantly different between daytime/nighttime-graded rain days. In the Tibetan Plateau, except for the significant decrease in the number of light rainy days at night, the rest of the rainy days of different magnitudes increased significantly during the daytime and nighttime, with the increasing trend at night being greater than that during the day, and the increasing trend of heavy rain and rainstorm at night was almost twice that of the day. Unlike the Tibetan Plateau, the number of days with light precipitation (light rain, moderate rain, and heavy rain) decreased significantly in Southwest China. Among them, the number of light rainy days at night decreased the most, approximately twice as much as during the daytime; the number of days with heavy precipitation (rainstorm or torrential rain) increased in the opposite direction, and the increasing trend at night is greater than that during the daytime.