Abstract: The formation and development of bottom undercutting area are one disease of soil sites, which is the most typical and common disease affected by environmental. This paper investigates, monitors and analyses the morphology, salinity, dry-wet conditions and wind field characters of the Great Wall on Ming Dynasty ruins in 12 cities along the Hexi Corridor. It finds that the morphology, salinity, wind field characters and temperature-humidity change in the process of rainfall are significantly different at the site of shady and sunny slopes of the undercutting area. Based on the results of wind tunnel simulation experiment, in which remodeling samples desalinated and mixed with different contents of NaCl and Na₂SO₄ and undergoing 3, 6, 9, 12 dry-moisture cycles, differential effect in the site of shady and sunny slopes of the undercutting area of earthen ruins in arid region is proposed. It also reveals the influences of salt type and content, number of wet and dry cycles, wind velocity and time of blowing on differential effect. Finally, the formation mechanism of differential effects is discussed with the results of investigation analysis and experiment.