Crack detection is of great practical significance for steel structures and concrete buildings, nuclear power, aviation and other fields. In this paper, a resonant coaxial probe is used to construct a microwave microscope to carry out the study of non-destructive testing methods for metal and non-metallic cracks. First of all, the influence of key parameters such as coupling gap, lifting height and probe material on the detection performance is studied through electromagnetic simulation. Then, the experimental verification was carried out with machined brass, aluminum alloy samples and 3D printed non-metallic samples. The results show that the S11 amplitude of the coaxial probe at the resonant frequency can be used for non-destructive detection of cracks on the surface of the material. The coupling gap and lifting height are the main design parameters that affect the detection sensitivity. Compared with the traditional reflective coaxial probe, the detection sensitivity of the resonant probe (S11 at the observation frequency point when there is a crack or not) The amplitude difference) has been increased by about 20~30 times; the simulation of the 3.9 mm crack in the width of the metal sample is in good agreement with the measured results. The relative error of the crack width detection is < 7.7%, and for 3D printed non-metallic crack samples with a crack width of 1.7 mm, the measured relative error of the crack width is Within 6%. This research has certain reference significance for the non-destructive detection of metal and non-metallic cracks.