The daily and annual temperature differences in northwest China vary greatly, which leads to thermal fatigue deterioration of concrete. Under the condition of constant environmental humidity, the thermal fatigue tests of concrete with two strength grades were carried out at the temperature difference of 20 ℃, 30 ℃ and 40 ℃ to measure the changes of the macroscopic properties such as compressive strength and splitting tensile strength. The microstructure changes of concrete were measured by ultrasonic nondestructive technology and mercury intrusion porosimetry test. The results show that the thermal fatigue degradation effect is obvious. With the increase of temperature difference and number of temperature difference cycles, the concrete strength decreases obviously. The decrease of C40 concrete is greater than that of C25 concrete, and the splitting tensile strength is more sensitive to thermal fatigue than compressive strength. The ultrasonic velocity decreases, which indicates that the internal defects of concrete increase. The porosity, total pore volume, average pore diameter, medium pore diameter and the most probable pore diameter of concrete increase with the increase of temperature difference and number of cycles, while the total pore surface area decreases, which indicates that the pore structure shows the characteristic of coarsening and a tendency to deteriorate. The porosity of C40 concrete is smaller than that of C25 concrete, but the relative change value of the porosity of C40 is larger, which reveals the internal reason for strength damage of concrete under thermal fatigue from the microscopic level.