Abstract: The top cover condition of railway subgrade may change from evaporation boundary to sealed boundary after frozen in winter time, which will influence the transportation of moisture up from the bottom and its accumulation beneath the frozen surface. The concept of ‘time-varying canopy effect’ is introduced and a mathematical model is established for its analysis based on the non-isothermal coupled water and heat migration model as well as the rigid ice model. Water migration in one-dimension soil column is simulated considering constant canopy effect and ‘time-varying canopy effect’ respectively. Simulation results show that the frost heave under time-varying canopy conditions is smaller than that under constant canopy conditions, but it may still be large enough to disturb the normal performance of the railway. The influences of soil type, temperature difference and initial water content on the ‘time-varying canopy effect’ are also discussed.