Glass fiber reinforced composite materials are widely used in aerospace, construction, and transportation industries. However, due to the defects such as debonding and delamination during the production and using processes of composite materials, non-destructive detection is necessary to carry out. In this paper, the method combining terahertz tomography and defect characteristic waveform imaging is used to study the debonding defects of glass fiber reinforced composite materials with different depths and different areas. Aiming at the fringe interference in tomography, a phase-based fringe suppression method for terahertz tomographic images is proposed, which eliminates the fringe interference in tomography, and further improves the imaging quality. The identification of debonding defects with a distance from surface of 5 mm, a diameter of 5 mm, and a thickness of 0.05 mm is realized. This study provides a new method for processing the results obtained by reflection type terahertz tomography.