【Objective】The Qinghai-Tibet Plateau is an area relatively active in ground-atmosphere interaction, Only through understanding in depth variation of soil thermal diffusivity in the Qinghai-Tibet Plateau, can the surface energy balance on the Plateau be correctly calculated and the impact of the Plateau on global and regional climate change be understood in depth. 【Method】Based on the data of soil temperatures at 0.8 m and 3.2 m deep in soil profiles accumulated in the 39 observation sites in the Qinghai-Tibet Plateau during the years from 1980 to 2001, thermal diffusivity of each observation site was worked out through fitting of the heat conduction convection method coupled with the least square method, and spatio-temporal variation of soil thermal diffusivity figured out. The heat conduction convection method encompasses heat conduction and thermal convection in calculating soil thermal conductivity. The amplitude and phase was obtained through fitting of the measured soil temperatures at the two soil depth with the least squares method, and substituted into the formula to yield soil thermal diffusivities. 【Result】 Results show that from 1980 to 2001, soil thermal diffusivity of the Qinghai - Tibet Plateau appeared to be on a declining trend. It fluctuated quite sharply before the 1990s and less in the 1990s. As affected by climate factors, soil properties and soil moisture content, soil thermal diffusivity vary in spatial distribution. Soil thermal diffusivity in the deep soil in the eastern part of the Qinghai-Tibet Plateau increased from spring to summer and decreased from summer to autumn and from autumn to winter. It peaked in summer up to 8×10^-6 m² s^-1 in summer at the junction of Qinghai Province, Sichuan Province and Gansu Province and in winter up to 5.1×10^-7 m² s^-1. In the rest of the Plateau, soil thermal diffusivity decreased from spring to summer, increased from summer to autumn and decreased again from autumn to winter, varying in the range of 1.2×10^-7 m² s^-1 to 9.2×10^-7 m² s^-1. For years the maximum mean monthly value of soil thermal diffusivity was observed in the eastern part of the Qinghai-Tibet Plateau, including Qinghai, Southwest of Gansu and West Sichuan, being 6.4×10^-6 m² s^-1, whereas the minimum was in the region of Qilian Mountains, being 1.2×10^-7 m² s^-1, and the mean monthly soil thermal diffusivity was relatively high in the central part of the Plateau. As affected by geographical location, soil properties, soil moisture content and number of snow coverage days, the central and eastern parts of the Plateau, to the northeast part of Nagqu, are sorted as alpine frigid humid climate zone, where the soil thermal diffusivity is high due to high precipitation and high soil water content, and the area of the Qilian Mountains is as alpine frigid semi-arid climate zone, where the soil thermal diffusivity is low varying in the range of 1×10^-7~4.8×10^-7 m² s^-1 because it is a permafrost region low in soil water content. In most parts of the Qinghai-Tibet Plateau, soil thermal diffusivity varies in the magnitude order of 10^-7 m² s^-1, and in the central and eastern parts abeing relatively high-valued regions, it does in the magnitude order of 10^-6 m² s^-1.【Conclusion】All the findings in this study demonstrate that soil thermal diffusivity varies sharply temporally and spatially, as affected by a complicated variety of factors, which merit further studies.