Abstract: The storage and release of heat in the soil of the Qinghai-Tibetan Plateau can trigger changes in weather，such as altering the trough-ridge allocation of the East Asian weather system. As a “buffer layer” between permafrost and the atmosphere，the active layer is sensitive to climate change，and responds quickly to temperature changes. The active layer controls energy and water exchange between the soil and the atmosphere，which results from changes in soil water content and temperature. To some degree，it reflects the thermal condition of the underlying surface，and therefore it can be used as a thermal indicator of the plateau surface. Therefore，the thermal regime of active layer in the permafrost regions is very important aspect in the research of land surface processes in cold regions. Thus more and more scholars focus their attentions on the thermal process of active layer. Limited by the observational data in situ，up to now it was not possible to exactly describe the thermal regime of freeze-thaw processes in active layer in the permafrost regions on the Plateau. Therefore，an in-depth study of the soil thermodynamic properties of the active layer is needed. In this paper，the characteristics of soil heat flux and soil heat balance coefficient were analyzed by using the radiant data observed from September1993 to October 2000 and the meteorological data measured from January 1961 to October 2010 at WDL weather station located in the northern Tibetan Plateau. The results showed that the soil heat flux took on a clearly inter-annual and inter-decadal variation features. The value of soil heat flux was less than 0.0  Wm-2 throughout the time period from 1960s to 1980s. Correspondingly，the active layer soil released heat to atmosphere during this time period. While since the 1990s，soil heat flux was greater than 0.0 Wm^-2，which showed that in a year，there was surplus heat amount transferred from the surface to the lower soil layer，and the active layer soil here mainly absorbed heat in this time period. Generally，soil heat flux presented an increasing trend during the past 50 years at a rate of 0.31 Wm^-2 per decade. Soil heat flux increased with the increase of surface net radiation．As for the soil heat balance coefficient，it presented the similar characteristic as the soil heat flux. The values of soil heat balance coefficient of active layer were less than 1.0 during the time period from 1960s to 1980s，which indicated that the permafrost here was relatively stable during this time period. While such values were greater than 1.0 from 1990s to 2010s，which meant that heat amount absorbed by soil was bigger than that emitted by the soil. As a result，the active layer thickness in the study region increased 4-5 cm in recent 12 years; annual surface soil negative cumulative temperature and positive cumulative temperature exhibited a rising trend at rate of 193.4 148.8 ℃d/10a and 148.8 ℃d/10a，respectively; and the number of frozen days of active layer decreased by 8.9 days per decade. Such phenomenon suggested that the frozen ground took on a degenerate trend in study region. Finally an empirically derived model was proposed for estimating the soil heat balance coefficient over the northern Tibetan Plateau. It could be expressed as the function of air temperature，surface soil temperature and surface water vapor pressure. Verification results further ensured that the proposed model predicts values of soil heat balance coefficient accurately.

Key words: river source region, soil heat flux, heat balance coefficient