Using Terahertz time-domain spectroscopy technology, the transmission spectra of polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene and acrylonitrile - butadiene - styrene are measured. Spectral characteristics of these five materials between 0.2~2.6 THz are studied.The absorbance and refractive index spectra of five plastic at room temperature in nitrogen atmosphere are obtained. The results show that refractive index and absorption coefficient of these five plastic are significant different,and the refractive indexes are in the ranges from 1.35 to 1.85. Compared to polyvinyl chloride and acrylonitrile-butadiene-styrene, the absorbance of polyethylene, polypropylene and polytetrafluoroethylene are much smaller. The research provides a method to identify plastic types and choose substrate of terahertz optical components using terahertz time-domain spectroscopy.