Abstract: The reflective terahertz time-domain spectroscopy was used to detect the glass fiber composite layer in the annular cut samples of the aluminum liner fiber fully wound high-pressure hydrogen storage cylinders. By collecting the reflection signal of terahertz wave inside the material, the time difference between the first and second reflection peaks was extracted, and the thickness distribution of the glass fiber layer was calculated by combining the refractive index of the material, which verified the feasibility of this technology in the interface identification of composite materials. This terahertz detection method could realize non-contact and high-resolution damage detection, which provided an effective technical means for manufacturing quality control, in-service detection and failure analysis of gas cylinders.