Abstract: This study investigates the climatological characteristics of the atmospheric heat source and cloud amount over Yunnan before and after the onset of summer monsoon rainy season using meteorological field of ERA5 (European Center for Medium-Range Weather Forecasts fifth generation global atmospheric reanalysis dataset) and satellite-retrieved precipitation and cloud amount data during 2001–2020. The results show the following: (1) The atmospheric heat source and cloud amount over Yunnan are strongly influenced by summer monsoon circulation and exhibit significant monthly variations. In June, the transport of water vapor by the southwesterly from the eastern Bay of Bengal causes Yunnan summer monsoon rainy season. After the onset of the summer monsoon rainy season, the regional atmospheric heat source (especially latent heat released by precipitation condensation) and cloud amount significantly increase with respect to those in April–May, while surface sensible heat flux and net radiation flux of air column are reduced. (2) According to the method reported by Wang and LinHo (2002), the average onset time of the summer monsoon rainy season over Yunnan is approximately pentad 31 during 2001–2020. The regional atmospheric heat source (latent heat) and cloud amount correlate well with the total precipitation rate. The changes in the atmospheric heat source and cloud amount with the summer monsoon rainy season over Yunnan present annual variations similar to those over the adjacent South tropical monsoon region, with a single peak structure. (3) The onset time of the Yunnan summer monsoon rainy season demonstrates an evident interannual variability. Synthetic analyses reveal that in the early-onset years, the low-level southwesterly wind from the southeastern Bay of Bengal can reach the Yunnan region while the high-level divergence exists, which is conducive to regional ascending motion, and atmospheric heat source (latent heat) and cloud amount are much higher than those in the late-onset years. The southern branch trough in the Bay of Bengal is weaker, and the position of the Western Pacific subtropical high extends more toward the west in late-onset years, which are unfavorable for regional ascending motion and precipitation over Yunnan. In the peak period of the Yunnan summer monsoon rainy season, the precipitation rate is similar in the early- and late-onset years, but the atmospheric heat source (latent heat) and high cloud amount in the late-onset years are slightly higher than those in the early-onset years.