Gas-insulated metal-enclosed transmission lines have the advantages of high capacity, low loss and reliability, and have the prospect of wide application. Currently, insulation failure is the primary cause of equipment failure. In this paper, combining the principle and test method of ultrasonic detection, the detection technology of defects and cracks of tri-post insulators based on ultrasonic guided wave is investigated. A platform for ultrasonic signal detection of tiny defects of insulators is constructed, and the reflective and refractive properties of defects to the ultrasonic guided wave conduction are verified within the COMSOL simulation platform, which proves the reliability of ultrasonic detection of crack defects. The EMD-wavelet thresholding multiple noise reduction algorithm is utilized to achieve post-processing, and the typical signal characteristics of different types of material defect models are analyzed. The signal-to-noise ratio and root-mean-square (RMS) evaluation indexes are introduced, obtaining a signal-to-noise ratio of 21.847 and a RMS error of 0.041, which are superior to other noise reduction methods. Ultrasonic detection of different types, materials and defects of materials, the two low peak distribution of epoxy matrix and silicone rubber containing defects is concluded, which provides theoretical basis and experimental guidance for subsequent ultrasonic insulation detection.