Abstract: Based on the propagation characteristics of microwave and the microwave sensitivity to the interface, the phase difference at the interface of thermal barrier coating(TBC) and waveguide probe was used to characterize the long crack on the metal substrate surface under TBC using computer simulation technology-microwave studio (CST-MWS). The influences of top coating thickness and crack direction were studied. Besides, microwave evaluation of cracks with different width and the crack length direction parrallel to the long brim of the rectangular waveguide probe were studied when the thickness of TBC is 400 μm. The results show that the top coating thickness affects the sensitive frequency, and that the carck direction influences the evaluation sensitive, and the evaluation sensitive is the minimum when the angle between the rack length direction and the long brim of the rectangular waveguide probe is 50°-55°. It is also observed that the crack can not be detected when the crack width is less than 8 μm, and the effect of detection is not obvious when the crack width is 10-30 μm, and the wider the cracks, the greater the phase difference of the reflection coefficient when crack width is in the range of 30 μm-1 mm. Therefore, it is feasible to evaluate the crack on the metal substrate surface using microwave non-destruction evaluation.