Abstract: A laser scanning thermal conduction model was established to accurately describe the characteristics of the sample surface temperature for laser scanning infrared thermography. It is concluded that the surface temperature variation is the same as the 1-D heat transfer model when the laser scanning speed reaches the appropriate range. The surface temperature field expression for limited thickness sample was obtained by using the mirror image source method. According to the law of reflection and transmission, double-layer structure temperature of laser scanning was obtained. Through numerical calculation of sample surface temperature field in different depth with different types of defects, the theoretical foundation to judge the defect depth and defect type was established for the laser scanning infrared thermography.