The stability of transmission line tower foundation soil is crucial for the proper functioning of high-voltage transmission lines. To better understand the mechanisms behind soil deformation and instability, soil samples were collected from a transmission line tower foundation in Guangzhou. Tests were conducted under different stress paths to examine the effects of loading and unloading on the mechanical properties of the soil. A constitutive model was developed to account for unloading effects. The results showed that the stress-strain relationship of the foundation soil exhibited normalized characteristics under loading conditions, with peak deviator stress increasing with confining pressure. Soil failure was predominantly characterized by bulging with minor surface cracks. When taking unloading effects into account, the soil initially underwent strain softening followed by strain hardening. The peak deviator stress at failure was lower than under loading, resulting in more severe soil failure. The study highlights the importance of considering the effects of excavation and unloading on foundation soil when constructing tower foundations, and implementing support measures in advance for stress compensation.