Fiberreinforced plastics have excellent properties such as strong corrosion resistance, good dielectric properties, and good thermal properties. They are widely used in aerospace, railway and other related industries. In this paper, the pulse-width modulated terahertz pulse excitation source is used as the input light source, and the measured optical parameters of materials in the terahertz band are fitted and optimized using the Debye model, which are used as the material input parameters in the finite-difference time-domain method. A terahertz pulse detection model for the bonding defects of fiber reinforced plastics is established. The finite-difference time-domain method is used to numerically analyze the propagation characteristics of terahertz waves in the bonding defects of fiber reinforced plastics. The propagation process of terahertz waves in the bonding defect structure and the causes of higher-order echoes are analyzed from the two aspects of time domain waveform and B-Scan imaging. We diagnose and analyze the debonding and delamination defects in fiber reinforced plastics. The thickness of delamination defects in 1 mm thick fiber reinforced plastics can reach 20 μm. The number of layers for layered defect detection with a thickness of 80 μm reaches two layers for fiber reinforced plastics. The proposed method can provide a theoretical support for the non-destructive detection and evaluation of bonding defects in fiber reinforced plastics based on terahertz pulse imaging.