Finite-difference time-domain and terahertz time-domain spectroscopy are used to study the internal delamination defects in glass-fiber-reinforced composite materials. First, the optical parameters of the glass-fiber-reinforced composite material in the terahertz frequency band are obtained using transmission terahertz time-domain spectroscopy; the finite-difference time-domain method is used to simulate the defects. Second, a terahertz time-domain spectroscopy system is used to inspect the prefabricated sample. Finally, the defects are imaged and analyzed for comparison to the time-domain waveform data determined theoretically and through actual detection. It is found that both methods can detect delamination defects with a thickness of 0.3 mm at a distance of 3 and 5 mm from the upper surface of the materials. These results show that the finite-difference time-domain method can provide theoretical support for detecting defects in composite materials by using terahertz time-domain spectroscopy while reducing the dependence on standard components; the terahertz time-domain spectroscopy system can effectively detect internal defects of materials and assess their overall performance through the detection results.