With the rapid development of power systems, the quality and performance of electrical insulation materials are critical to ensuring the safety of inspection personnel. Traditional inspection methods, such as ultrasound and X-rays, have certain capabilities for defect detection but face limitations in terms of detection accuracy, penetration depth, and non-destructive testing. Terahertz waves, with their unique frequency range (0.1–10 THz), offer significant advantages in penetration, resolution, and safety, making them an emerging technique for detecting internal defects in electrical insulation materials. This paper proposes a method for detecting defects in electrical insulation materials using a terahertz focal plane imaging system. The method leverages the non-ionizing properties of terahertz waves to perform non-destructive internal inspections, accurately locating defects at depths of up to 0.5 mm and detecting features as small as 1 mm. The study demonstrates that the terahertz focal plane imaging system can efficiently detect internal air gap defects and precisely determine the location and size of these defects. This technology shows great potential for application in defect detection for electrical insulation materials and promises to provide important safeguards for the safety of power equipment inspection personnel.