Abstract：The thermal regime of soil is vital for indicating the presence and thermal stability of permafrost. To investigate the long-term changes in the thermal regime of permafrost in the Headwater Area of the Yellow River (HAYR), we first constructed a mathematical model for soil heat transfer to simulate the dynamics of ground temperatures at six boreholes using the HYDRUS-1D model. The reliability and applicability of the model were confirmed through parameter calibration procedures. Subsequently, the changes in permafrost thermal regime from 1979 to 2018 in the HAYR were simulated using monthly air temperature data extracted from the China Meteorological Forcing Dataset (CMFD). The model simulations revealed an abrupt change in mean annual ground temperature in the HAYR after 1999. Prior to 1999, the changing rates were from ?0.037 to 0.026 °C/a, whereas after 1999, they ranged from 0.006 to 0.120 °C/a. The abrupt increase in mean annual air temperature in 1998 and the occurrence of extreme climate disasters in 1999 were identified as the primary reasons for the sudden changes in permafrost thermal regime in 1999. The rise in permafrost temperature and the decrease in its thermal stability are expected to have various impacts on the water resources conservation function and biogeochemical cycle. This study can provide scientific and technological support for clarifying the response patterns of plateau permafrost to climate change and for strengthening zoning and control of the ecological environment in the HAYR.