Abstract: In order to discriminate crack defects in base material covered by spray coatings, the heat generation pattern of coating crack and substrate crack were investigated when being excitated by low frequency ultrasonic vibrancy, based on theoretical analysis and numerical simulation. The surface temperature varieties caused by coating crack and substrate crack were compared; the turbulence to coating crack thermal wave caused by heat generation of a substrate crack was analysed, and the phase deviation (PD) phenomenon of mixed thermal wave was observed. On this basis, a method to identify cracks in the base material under the spray coating was proposed, which employed phase deviation as the characteristic parameter. Furthermore, experiment was implemented to validate the proposed method, polynomial fitting (PF) and wavelet decomposition (WD) were utilized to improve the time frequency resolution ratio during image processing and reconstruction.