Power insulators are widely used in the electric power field owing to their high mechanical strength, crack resistive surface, and slow aging speed. A terahertz wave reflection propagation model with three layers of power insulators is constructed to study the air gap between layers of power insulators. An air gap defect sample of rubber-epoxy resin plate was fabricated by simulating the air gap between layers of power insulator. The waveform analysis of the samples containing the interlayer air gap defects reveals that the thickness of the interlayer air gap defects is 100 μm. Moreover, the thickness of the interlayer air gap is calculated. The accuracy of THz detection of the interlayer air gap thickness is 95.61%, as verified by a digital display thousand meter. The energy integral imaging was performed on the defect samples and the maximum between-class variance (Ostu) binarization method was used to extract the air gap defects. To verify the recognition accuracy of THz detection, the optical coordinate measuring machine (CMM) 3D was used to reconstruct the bonding defects and extract the defect area, and the detection accuracy is 95.29%. This study provides a new method for detecting interlayer air gap defects in power insulators.