Abstract: Cross-linked polystyrene is a kind of thermosetting plastic with excellent dielectric properties and high breakdown voltage. We measured the terahertz spectra of cross-linked polystyrene and polystyrene using THz-TDS based on THz-ABCD(air biased coherent detector), and the absorption coefficients and refractive indexes in the range of 2-13 THz were obtained. The terahertz spectra of cross-linked polystyrene and polystyrene were similar, and there existed several absorption peaks such as 6.6 THz, 9.8 THz and 12.25 THz, the absorption coefficient of cross-linked polystyrene was higher than that of polystyrene below 12 THz but lower above 12 THz, the refractive index of cross-linked polystyrene (1.59-1.62) was higher than that of polystyrene (1.58-1.59).