The temporal and spatial characteristics of precipitation over Southwest China and the surrounding regions were analyzed using observation data from 124 stations in Yunnan Province and high-resolution Climatic Research Unit precipitation data. The precipitation was analyzed for summer and autumn. The relationship between precipitation and monsoon activity, and the circulation background during drought and flood periods were also analyzed to investigate the possible mechanisms affecting the decadal variabilities. The results show that the spatial distribution of precipitation in these areas evolve with the seasons, with a relatively small value over Southwest China. Precipitation in all seasons has obvious interannual variability and interdecadal oscillation characteristics. By further analyzing the decadal-scale period of precipitation in summer and autumn, this study shows that the continuous drought that has occurred in Southwest China in recent years is likely caused by the decadal-scale periodic negative phase cooperation of precipitation in summer and autumn. In addition, the precipitation reduction is associated with the weaker summer monsoon activity and its shorter duration. There are significant differences in the atmospheric circulation during drought periods and flood periods in summer and autumn, respectively. Anomalous northerly winds appear in the lower troposphere during the drought period, and anomalous anticyclonic circulation occurs around the Tibetan Plateau in the upper troposphere, and vice versa.