Field survey, systematic sampling and laboratory tests were conducted to study the thermal deterioration of superficial layer on the rammed earthen ruins, taking the Wuwei Ming Great Wall at Gansu in China as an example. X-ray diffraction and SEM observation indicate that the superficial layer is characterized by obvious weathering and smoother surface when compared with the host soil. Thermal deterioration simulation test shows that the thermal conductivity coefficient, thermal diffusion coefficient and specific heat of volume in the superficial layer are lower than those of the host soil. The conductivity coefficient and thermal diffusion coefficient keep relatively stable during the thermal deterioration simulation process. It is concluded that, under the effect of temperature variation, interface thermal stresses are generated due to the thermal-parameter difference between the superficial layer and host soil, and then the sustaining action of thermal-stress difference results in the fatigue failure and detachment of superficial layer.