Abstract: With the ongoing transformation of China's energy structures, solar thermal power generation is playing an increasingly vital role. Small piles are often used as the foundation of heliostat in solar thermal power station. In expansive soil regions, the swelling behavior of foundation soil under moisture absorption induces uplift forces on heliostat pile foundations, threatening structural safety. To investigate this mechanism, laboratory expansion tests were conducted to establish the swelling ratio-vertical pressure relationship. An elastic theoretical solution for single-pile load transfer was derived via the load transfer method, integrating time-dependent pile-soil interaction and soil expansivity. The validation through large-scale immersion model tests revealed that under swelling conditions, upward tensile forces dominate the upper pile segment, while anchoring forces act on the lower segment, resulting in a non-monotonic axial force distribution peaking at the neutral point. Prolonged immersion caused downward migration of the neutral point along the pile shaft. A minimum pile length was thusly designed and calculated to counteract uplift displacement through anchorage enhancement, so as to provide a theoretical guidance for pile foundation design in solar thermal collector fields within expansive soil areas.