Abstract: In summer, the air temperature on the Qinghai–Xizang Plateau experiences intraseasonal variations characterized by initial warming followed by cooling. Therefore, studying the long-term changes in intraseasonal air temperature can improve the understanding of the long-term air temperature changes over the plateau. This study employed the JRA55 and ERA5 reanalysis datasets to analyze the long-term warming trend of summer air temperatures in the eastern Qinghai–Xizang Plateau, focusing on its intraseasonal evolution and the underlying atmospheric thermodynamic mechanisms. The results show that the warming trend of the summer temperature over the eastern Qinghai–Xizang Plateau is primarily related to three factors: (1) the significantly accelerated warming rate in mid-June and late June, (2) the significantly decelerated cooling rate in mid-August, and (3) the significantly prolonged warming period. Thermodynamic diagnostic results indicate that the meridional temperature advection, which plays a cooling role climatologically, shows a long-term weakening trend. This weakening trend may be an important cause of the warming of summer temperatures over the eastern Qinghai–Xizang Plateau. Notably, while both reanalysis datasets generally revealed consistent air temperature evolution over the eastern Qinghai–Xizang Plateau, they differ in the thermodynamic budget equation, with the JRA55 dataset providing better diagnostic results.