HanYangling is China’s first underground museum having a constant temperature and humidity. Nevertheless, a layer of hydrated calcium sulphate(CaSO₄·2H₂O)has appeared on the surface of the site. Based on longterm monitoring data of Pit 13, including the environmental temperature and humidity,evaporation, the soluble salts at the site and groundwater, the appearance of the calcium sulphate layer is tried to be understood.Pit 13 has a yearround environmental humidity of around 100%; the site surface temperature is higher than the dew point. Since water molecules in the air can not be transformed to liquid water on the soil surface and return to the soil, the concentration of soil solutions increases when the soil moisture evaporates. As a result, the ions in the soil reacts with each other until white calcium sulfate (CaSO₄·2H₂O) appears on the surface of the site.