As a new advanced composite material, glass fiber reinforced composite material has the unique advantages of light weight, high temperature resistance and impact resistance, which are widely used in aviation, military industry and other fields. However, during its production process, due to the influence of the manufacturing process, it is easy to produce micro defects such as delamination and inclusion inside material; on the other hand, in the life cycle of such composites material, due to the interference of external factors such as impact force and high temperature, there are defects such as burning marks and debonding on the surface and inside of the materials. The existence of defects reduces the safety factor of glass fiber-reinforced composites material as structural parts, so it is necessary to detect the defects in the materials. Terahertz time-domain spectroscopy benefits from the unique advantages of terahertz band as an effective supplement to the traditional nondestructive testing methods, terahertz time-domain spectroscopy has been widely used in the field of composite nondestructive testing in recent years because of its transient, low energy and fingerprint spectrum, which can effectively detect defects. The performance of glass fiber reinforced composites material is evaluated through the test results. However, when using terahertz time-domain spectroscopy to detect and analyze defects, it is found that some fringes diffuse with time in the tomography process of defects, the existence of fringes masks the shape of defects, affects the clear identification of defects, and further leads to missed and misjudgment of defects. There is little research on the analysis of fringe in defect tomography in theory. This paper proposes the finite-difference time-domain technology to model and analyze the interaction mechanism between terahertz wave and glass fiber reinforced composites material. A reflective numerical model is established in the frequency range of 0.2~1.5 THz. Through numerical simulation, the defects at the depths of 1, 3 and 5 mm in glass fiber reinforced composites material are imaged. It is also found that when the terahertz wave is vertically incident on the surface of the glass fiber reinforced composite material, the defects at each depth can be imaged, when the terahertz wave is incident on the surface of the glass fiber reinforced composite material at an inclination angle of 1°, and there is a 2° inclination on the upper and lower surfaces of the glass fiber composite material, alternating fringes appear in the defect imaging at each depth inside the material, it is verified that the fringes are caused by interference.