Abstract: To reveal the strength variation and shear failure characteristics of karez soil in the Turpan region, the field investigations and a series of direct shear tests are conducted under different water contents and temperatures. The internal shear mechanisms are also analyzed through the scanning electron microscopy (SEM). The results show that: (1) The shear strength of karez soil in the Turpan region is closely related to the water content and temperature. As the water content increases, the shear strength of sandy soil tends to decrease. (2) With the increasing temperature, the shear strength of sandy soil first decreases and then increases. The SEM reveals the microscopic mechanisms by which the change of the water content affects the shear strength of sandy soil. When the temperature is below 0℃, as the temperature rises, the water in the soil transits from ice phase to liquid one, which reduces the adhesive force between particles, thereby decreasing the shear strength. When the temperature is above 0℃, further evaporation of the water in the soil increases the matric suction, causing tighter particle-particle contact and enhancing the physico-chemical effects of the water film, thus increasing the shear strength.