A physically based analytical model is formulated to simulate the thaw depth of active layer under changing boundary condition of soil heat flux. The energy conservation statement leads to a nonlinear integral equation of the thaw depth using an approximate temperature profile as an analytical solution of the diffusion equation describing the heat transfer in the active layer. The time-varying soil surface heat flux is estimated using non-gradient models when field observations are not available. The proposed model was validated against field observations at three Arctic forest and tundra sites. The simulated thaw depth and soil temperature profiles are in good agreement with observations hinting the potential for model application at larger spatial scales.