Thermokarst lakes on the Qinghai-Tibet Plateau cause considerable thermal disturbance to the surrounding permafrost, giving rise to anomalous ground-temperature conditions and open-talik formation below some lakes. Using in situ data and information from monitoring of a representative thermokarst lake in the Beiluhe Basin, this study simulates the rate of talik development beneath the lake, the time taken for an open talik to form, and the rate of permafrost lateral thaw after open-talik formation. The simulation uses a simplified two-dimensional unsteady finite-element model for heat transfer with phase change under a cylindrical coordinate system. The results indicate that a bowl-shaped talik forms under the lake and that the talik thickness increases substantially over time. An open talik forms below the lake 733 years after the lake formed. The average maximum thaw rates at the top and bottom of permafrost beneath the lake before the open talik forms are 5.8 cm year-1 and 0.7 cm year-1, respectively. By 1100 years after lake formation, permafrost beneath the central deep pool and shallow nearshore zone of the lake thaws completely and the heat-source effect caused by the lake becomes very limited. The volume of the open talik beneath the lake still increases gradually with time 1500 years after lake formation, but the increase is very limited. Copyright © 2012 John Wiley & Sons, Ltd.