Abstract: Xi'an Q₃ undisturbed loess was chosen to artificially prepare saline undisturbed loess specimens with different gradients of sodium sulphate content. The triaxial shear and CT scanning tests under freeze-thaw action are then conducted to study deterioration behavior and damage mechanism of microstructure of the saline undisturbed loess with sodium sulphate. The results show that the freeze-thaw action has no obvious effect on the type of stress-strain curves, which constantly present characteristics of strain hardening. The failure deviator stress decreases with the increasing number of freeze-thaw cycles while the attenuation rate gradually declines, indicating that freeze-thaw action results in weakening deterioration rate. Moreover, the failure deviator stress experiences a linear or growing decline rate at higher salt content, demonstrating that salt erosion leads to linear or strengthening deterioration rate. The cohesion exhibits the similar characteristics of deterioration with the failure deviator stress. The internal friction angle shows a little variation and no explicit regularity. The ratio of freeze-thaw deterioration factor to that for salt erosion increases with a growing number of freeze-thaw cycles, while its growth rate declines. In addition, the ratio decreases with higher salt content and its attenuation rate also declines. The ME value of CT scanning presents the similar variation with failure deviator stress and cohesion. A formula for microscopic damage variable is then established, and can well predict the damage rules of microstructure of specimens under freeze-thaw action. Both the macroscopic and microscopic damage variables show the consistent variation, indicating that microscopic damage variable well reveals the deterioration mechanism of triaxial shear strength indexes.