Abstract: Using 20th Century Reanalysis (20CR) data, this paper analyzes the spatial and temporal characteristics of long-term trend of air temperature and depicts the main time period of air temperature in Southwest China based on the spectrum analysis. Our results have five points. First, the air temperature of Southwest China is warmer in its south region (23°—26°N, 101°—107°E) in winter, and it is warmer in its southwest region (22°—25°N, 98°—102°E) in spring. Second, there is an opposite spatial pattern of long-term trend of air temperature between summer and autumn. That is, the long-term trend is generally decreasing from west to east in summer, while it is increasing from west to east in autumn. Third, the warming trend of Southwest China is overall more significant and its corresponding area is overall larger in summer and autumn than in winter and spring. Fourth, the strongest long-term trend of the air temperature is located in the key region of the east of Southwest China during autumn (23°—32°N, 104°—109°E), and the weakest long-term trend of the air temperature is located in the key region of the east of Southwest China during summer (23°—32°N, 105°—108.5°E). Fifth, the time series associated with the key region during four seasons mainly exhibits significant periods of more than 20 years.