Terahertz waves demonstrate exceptional penetration capabilities in non-polar materials, with terahertz echo imaging generated by internal dielectric discontinuities offering high resolution and non-contact operation. These characteristics position terahertz technology as a research priority for field-based non-destructive testing of insulation components like high-voltage ceramic bushings. This study introduces theoretical models for terahertz wave transmission and reflection, presenting systematic methodologies for extracting material dielectric parameters using both transmission and reflection approaches. Through scanning imaging, the thickness distribution of the anti-pollution flashover coating on the external surface of the high-voltage ceramic sleeve specimen is detected, and the prefabricated gap defects inside the ceramic are also identified. This research demonstrates terahertz imaging's dual capability for surface coating evaluation and subsurface defect detection in high-voltage insulation components.