The Three Rivers Source Region (TRSR) is an important water source and ecological reserve in China, and analyzing the variation characteristics of its freeze-thaw index provides a scientific basis for assessing the local permafrost environment and its response to climate change. In this study, the temporal and spatial characteristics of the air freezing and thawing indices in the TRSR from 1979 to 2022 were analyzed using daily air temperature data from a high-resolution near-surface meteorological forcing dataset for the Third Pole region, applying the air freezing and thawing index and other methods. The results show that the mean freezing index in the TRSR over the past 44 years was 1 930.23 °C·d, exhibiting a spatial pattern of gradual decrease from west to east. By contrast, the thawing index displayed an opposite spatial pattern with an average value of 879.25 °C·d. Overall, the freezing index in the TRSR has shown a fluctuating downward trend at a rate of -10.01 °C·d/a over the past 44 years, with an abrupt change in 2001, whereas the thawing index has exhibited a fluctuating upward trend at a rate of 6.29 °C·d/a, with no significant abrupt change. Altitude, a key factor influencing freezing and thawing indices in the TRSR, showed significant correlations. For every 100 m increase in altitude, the thawing index decreased by approximately 87 °C·d, whereas the freezing index increased by approximately 107 °C·d.