Permafrost degradation is prevalent on the Qinghai-Ti-bet Plateau. This may lead to changes in water and heat transition in soils and thus affect the structure and function of ecosystems. In this paper, using the measured data of alpine steppe in Wud- aoliang assessed the model performance in simulating soil freezing and thawing processes. Comparison of the simulated results by simultaneous heat and water (SHAW) model to the measured data showed that SHAW model performed satisfactorily. Based on analyzing the simulated and predicted results, two points were obtained: (1) freezing and thawing of the active layer proceeded both from the soil surface downward. Compared with the freezing process, the thawing process was slower. The freezing period persisted in the surface layer (4 cm depth) for about 5 months; (2) in the next 50 years, frozen period would be shorten about 20 days in the top 100 cm depth while the thawing would start earlier 40 days than present. Soil water storage in the 0-60 cm would de- crease by 22% averagely, especially from June to August when the vegetation is at the dominating water consumed stage. Therefore, this kind of permafrost degradation as active layer freezing and thawing processes changes will reduce soil water content and thus influence those ecosystems above it.