Abstract: To identify the mechanism of ultrasonic infrared thermal image detection of concrete crack detection, the concrete slab specimens with prefabricated cracks were designed, and the thermal test of concrete slab cracks under ultrasonic excitation was carried out by using the ultrasonic infrared thermal image detection system. By analyzing the infrared thermal image of the crack heating and the microscopic image of the crack surface after the test, the heating mechanism of the concrete slab crack under ultrasonic excitation was summarized. Based on the heat generation mechanism of cracks, the effects of pre-tightening force, excitation time and excitation intensity on the crack heating were analyzed. The test results indicate that the crack heating originates from the contact, collision and slippage of the crack surface. The middle and lower parts of the crack surface are the main heat generation areas, and the crack surface is flat and without filler. Increasing pre-tightening force, excitation time and excitation intensity can input more ultrasonic energy into concrete slab specimens, thus promoting the increase of crack heating. When the pre-tightening force is 110-120 N and the excitation intensity is 21%-24%, the cracks have good thermal response, which is conducive to detection and identification.