Abstract: The capillary process is the main way of salinized deterioration effect and the important premise of aeolian ullage effect in undercutting areas at the bottom of earthen ruins in arid regions. By analyzing the real-time monitoring data of changes in the moisture content in the ground and undercutting areas at three typical earthen sites under normal weather in four seasons, rainfall and snow conditions, it is found that the rainfall (snow) weather is the inducing factor for the capillary process in the undercutting areas. Combined with the laboratory capillary simulation experiment, it is shown that the plastic limit of soil is the critical condition for the capillary process in the undercutting areas. Then based on the sampling and simulation experimental results of undercutting areas of 15 earthen ruins with different ages in arid regions, a theoretical model for the maximum height of capillary water rise is established by introducing the variables such as capillary water absorption, evaporation rate, etc. The calculated results by the theoretical model show that the absolute errors are less than 1 cm when the error correction coefficient is ε=0.9. The above researches provide important theoretical support and reference for the development mechanism and prevention of diseases in undercutting areas at the bottom of earthen ruins in arid regions.