Abstract: Ultrasonic lock-in thermography is a nondestructive testing method, which combines ultrasonic excitation and infrared lock-in thermography. The detection of contact interface defects of sheet metal was studied in this paper using ultrasonic lock-in thermography, and the selective heating advantage of the ultrasonic excitation was analyzed. The finite element simulation model of heat transfer in sheet metal was discussed, and the effect of ultrasonic excitation parameters on the detecting results was illustrated. Experiments were performed on prefabricated defects of sheet metal to validate the built simulation model. The result shows that amplitude decreases and phase is invariant when location of loading departs from crack, amplitude increases and phase is invariant when ultrasonic excitation amplitude increases, amplitude decreases and phase increases when modulated frequency increases. The built finite model accords with the experimental results. Ultrasonic lock-in thermography is fit for test surface defects of sheet metal, and provides guidance for ultrasonic lock-in thermography testing.