Abstract: When the surface of the unsaturated soils is covered by a water and air proof layer, the water content of the shallow soils will increase with time. This phenomenon is considered as the “pot-cover effect”. The analysis shows that the initial water content, boundary temperature and insulating layer have an important influence on the “pot-cover effect”. Here one type of the “pot-cover effect” is studied, in which the water supplement of the shallow soil depends on the water vapor transfer. A large number of numerical experiments are carried out to explore the influence laws of the initial water content, boundary temperature and insulating layer on the “pot-cover effect”. The research shows that when the initial water content of soils is close to the plastic limit, the maximum value of the liquid water content in the shallow soils during the low-temperature period is higher than that of other samples with different initial water contents, which is near the liquid limit. When the initial water content of soils is near the liquid limit, the increasing phenomenon of water caused by the “pot-cover effect” will disappear. This point corresponds to the vanishing critical point. With the decrease of the boundary temperature, the maximum value of the liquid water content in shallow soils during the low-temperature period increases at first and then decreases because of the moving down of the freezing front. The insulating layer in soils can cause “double pot-cover effect” and the water increment caused by the “pot-cover effect” can be significantly reduced by laying the insulating layer at the position where the soil temperature is around 0℃ (above zero).