Abstract: In the detection of composite insulation structures in electrical equipment, detection methods based on physical quantities, chemical components, and microscopic characteristics such as sound, light, electricity, and magnetism have been widely applied; however, it is difficult to achieve non-destructive visual characterization. In contrast, terahertz detection technology utilizes the optical and electromagnetic properties of terahertz waves to effectively reverse the structural and dielectric characteristics of the tested object, which has significant advantages over traditional detection methods in terms of non-destructive and online measurement. Therefore, this paper summarizes the current research status of terahertz non-destructive testing technology in the detection of composite insulation structures in electrical equipment. Firstly, the principle and system of terahertz detection method were systematically introduced, and a comparison was made between terahertz detection method and traditional detection methods. Secondly, the practical applications of terahertz time-domain and frequency-domain spectroscopy and imaging techniques in the aging and defect detection of composite insulation materials were summarized, including defect identification and localization, component identification, aging state evaluation, etc. Finally, the development prospects in terahertz non-destructive testing technology in this field were discussed. The authors hope this review can help relevant researchers quickly understand the research and application of terahertz non-destructive testing technology in electrical equipment condition diagnosis, and provide solutions to similar problems.