Abstract: The heat exchange capacity of the energy piles system in unsaturated soil cannot be accurately estimated, without considering the soil surrounding piles as a multiphase medium to include the coupled thermal-moisture migration. A numerical model of unsaturated soil considering thermal-water-vapor migration was established, and a one-dimensional soil column test was designed and conducted. The reliability of the model was verified using the monitored spatiotemporal evolution of temperature and humidity. Furthermore, the model provided an insight into the microscopic mechanism and thermal efficiency of energy piles. The results show that gravity has a significant impact on the migration of liquid water in vertical soil columns, while water vapor migration is mainly driven by temperature gradients. Under conditions of low water content and significant temperature difference, the water vapor flux can reach 17.0% of the liquid water flux. Neglecting the coupled thermal-moisture migration of unsaturated soil will overestimate the heat exchange performance of energy piles under the summer cooling mode and underestimate it under the winter heating mode, especially for soils with high water content and high coarse-grain content. The model and conclusions of this paper can provide a reference for the design of energy pile systems and the revision of relevant standards.