The interaction mechanism between terahertz waves and pore defect in glass fiber composites is explored, and the interaction relationship between porosity and terahertz characteristic parameters (refractive index, extinction coefficient and transmission coefficient) is analyzed at 0.075 and 0.713 THz frequency points. The experimental results show that the density and refractive index of glass fiber composites decrease with the increase in porosity. When the frequency is 0.075 THz, the extinction coefficient decreases and the transmission coefficient increases with the increase in porosity based on Rayleigh scattering theory. At 0.713 THz, based on Mie scattering theory, the extinction coefficient increases and the transmission coefficient decreases with the increase in porosity. In addition, the complex and changeable pore morphology results in a non-unique correspondence between porosity and terahertz characteristic parameters. When the different samples have the same porosity, the terahertz characteristic parameters are not the same.