Based on long-term monitoring of the diseases of the earthen site, temperatures and humidity in the museum, water contents at different depths, and also on measurement of the permeability coefficients and hardness of soil samples, it is found that the repeated occurrence of flooding, spalling and serious mould in winter and spring is related to the periodic change of moisture in soil by the “unidirectional evaporation” in a relatively closed environment, and that the direct damage of piping in the earthen site is caused by the seepage due to lawn watering, rainfall and urban drainage into the site. The mechanism of soil damage caused by groundwater changes was analyzed and primary measures to prevent drastic groundwater changes of the earthen site were advanced. This study aims to provide a reference for the conservation of the earthen site of Xi’an Tang Hanguang Gate Museum.