Frost heave has caused serious distresses to railway roadbeds in seasonal, especially deep seasonal, frozen regions. Therefore, frost heave prevention is the basic principle of roadbed design in seasonal frozen regions for many years. Frost heave has caused serious distresses to railway roadbeds in seasonal, especially deep seasonal, frozen regions. Therefore, frost heave prevention is the basic principle of roadbed design in seasonal frozen regions. The coupled heatfluid transport equation and twodimensional finite element method were employed to simulate the maximum frozen depth in the coming 50 years under the roadbed with XPS insulation board in the conditions of different depth, width, thickness and construction season for studying the insulation effect. Based on numerical simulation, appropriate depth, width and construction season were proposed for insulation design and the relationship between the thickness of insulation board and the height of roadbed was analysed as well.