Defects in the paste structure inside a workpiece act as a crucial factor affecting production quality and operational safety, and infrared non-destructive testing can be used to detect and evaluate defects. In this study, the blind frequency of the paste structure at different defect depths and thermal diffusivity of the coating were measured through simulations, and the influence of the defect depth and thermal diffusivity of the coating on the blind frequency was studied. Relationship between the defect depth and thermal diffusivity of the coatings. The simulation results show that the thermal diffusion length can be obtained using the blind frequency, and a quantitative detection method for the defect depth can be obtained.