In order to reveal the heat generation mechanism of crack during ultrasonic infrared thermography testing, an experiment was finished about testing a crack specimen when preload force was 100 N, 150 N and 200 N respectively based on the influence of preload force on heat generation of defects, and some results were obtained. It was observed that preload force was a direct ratio of the temperature evolution at crack field. Besides, the heat generation at both ends of the crack was significantly higher than the middle section, the hot spot in tip was most obvious, and the circle hot spot was most bright. Based on single degree of freedom damped system by displacement excitation and heat source temperature field superposition principle, a simple mathematical model for heat generation of crack under ultrasound excitation was proposed. After the temperature evolution of the reference point P■ and the crack tip point P■ was calculated by the theory model, it was found that the temperature evolution of P■ was consistent with the experiment result, and the error would decline between the temperature evolution of P■ and the temperature evolution curves and the temperature evolution rise was consistently between the both when the preload force increased. This model can describe the heat generation process at crack and provide a model foundation for the testing parameters optimization of ultrasonic infrared thermography testing, with certain theoretical significance and engineering value.