Cross-linked polystyrene is a kind of thermosetting plastic which has excellent dielectric properties and high breakdown voltage. It has great potential application value in microwave window, electrical engineering and military industry, while the infrared optical property of cross-linked polystyrene was rarely studied. A new cross-linked polystyrene sample was prepared by a novel method of γ-ray radiation polymerization in this paper. Cross-linked polystyrene and polystyrene were analyzed by Fourier transform infrared spectroscopy(FTIR), and the reflective spectra were measured. Subsequent Kramers-Kronig(K-K) analysis provided the dispersion of complex refractive index across the measured region, and calculation errors were analyzed. The results show that the refractive index of cross-linked polystyrene is similar to that of polystyrene in terahertz frequency, and in infrared region decreased rapidly with respect to the frequency(about 1.1-1.2 in mid-infrared region), which is much lower than that of polystyrene (around 1.5).