The soil thermal regime of the Tibetan Plateau is modeled by applying a one-dimensional heat transfer model with phase change. The two main forcing variables are surface air temperature, from the ERA-40 reanalysis, and snow depth, derived from passive microwave satellite data. Daily fields of soil temperature are simulated, ranging from the soil surface down to 30 m depth, and the horizontal grid cell resolution is 25 km × 25 km. Results are presented for three different soil moisture regimes. The trend analysis is based on daily fields of active-layer depth (ALD) for the period January 1980 through December 2001. Significant positive ALD trends for all Tibetan permafrost regions are simulated in response to positive air temperature trends, with the strongest trend for the northern Tibetan Plateau ( 1.4 cm yr⁻¹). Significant trends can reach 4 cm yr⁻¹ locally. The day of year when ALD was reached shows strong interannual variation, and significant trends occur for smaller areas than for ALD. As an application, the active-layer deepening is presented along the tracks of the Qinghai-Tibet railroad line that has recently been completed.