In this study, in order to explore the ability and experimental parameters to detect the internal defects of radar absorbing coating using the lock-in thermography technology, the method, which combined the finite element (FE) simulation and experimental research, is used to study for the square and circular simulated defects testing of the coating specimens. The analysis results show that the finite element simulation provides theoretical support for experiment and the defect of the equivalent area which is 3 mm×3 mm can be detected when the range of modulation frequency is 5 to 10 mHz. In addition,the phases of defects are lagged compared to defects cerea, and phase delay get bigger with the area of the defects increasing.