Spectral characteristics are obtained for Polyphenylene sulfide (PPS), Polyetherimide (PEI) and glass-fiber reinforced composites. First, by using terahertz time-domain spectroscopy system, time-domain signal of four samples are detected in free space for transmission mode. Then, according to the physical model and Fresnel formula extracting optical parameters, refractive index and extinction coefficient are calculated. Meanwhile, simulation analysis ensures the rationality and reliability of the test and calculation. Finally, based on the theory of error diffusion, error caused by crucial factors is exactly computed. Experiments show that in samples’ respective terahertz effective spectrum, PPS’s optical constants appear at n=1889~1945 (error=0003~0012), κ=0001~0047 (error=0000 1~0002 6); PPS-GF30’s optical constants appear at n=1654~1672 (error=0003~0004), κ=0001~0047 (error=0000 1~0002 8); PEI’s optical constants appear at n=1713~1733 (error=0002~0012), κ=0005~0035 (error=0000 1~0003 0); PEI-GF30’s optical constants appear at n=1688~1732 (error=0003~0004), κ=0036~0068 (error=0000 2~0002 6). The results indicate that, as a base of terahertz meta-material device, PPS is suitable for low frequency, but PEI is just the opposite. Compared with the pure samples, glass fiber reinforced PPS and PEI have been improved not only in mechanical properties but also in signal detection. In addition, detection sensitivity is stronger in the high frequency range. The study provides basic optical parameters of PPS, PEI and glass fiber reinforced materials, and it is also an important reference for meta-material device research in terahertz field.