This study investigates the climatological characteristics of atmospheric heat source and cloud amount over Yunnan before and after summer monsoon rain onset using ERA5 reanalysis and satellite-retrieved precipitation and cloud amount data during 2001-2020. The results show that: (1) atmospheric heat source and cloud amount over Yunnan are strongly affected by the summer monsoon circulation and exhibit significant monthly changes. In June, due to the transport of water vapor by the southwesterly from the eastern Bay of Bengal, Yunnan summer monsoon rain erupts. After the summer monsoon rain onset, regional atmospheric heat source (especially latent heat) and cloud amount increase significantly relative to those in April-May, while surface sensible heat and atmospheric radiative cooling decrease. (2) Based on the method by Wang and LinHo (2002), the average onset time of the summer monsoon rain 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 rain over Yunnan show similar annual variations as that over the adjacent South tropical monsoon region, with a single peak structure. (3) The onset time of Yunnan summer monsoon rain exhibits obvious interannual variability. The synthetic analysis shows that in early-onset years, low-level southwesterly from the southeastern Bay of Bengal can reach the Yunnan region, where the dominant divergence flow is conducive to the ascending motion and atmospheric heat source (latent heat) and cloud amount are much higher than those in 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 westward in late-onset years, which is unfavorable to ascent and precipitation over Yunnan. In the peak period of Yunnan summer monsoon rain, the precipitation rate is relatively close in early- and late-onset years, but atmospheric heat source (latent heat) and high cloud amount in late-onset years are slightly higher than those in early-onset years.