The complete equation of state ofpolystyrene was established based on the Helmholtz free energy model with describing the state before and after the dissociation of polystyrene respectively. The equation of state was validated by using the physical quantities of Hugoniot and Lagrangian sound speed which obtained from the magnetically driven shock and re-shock experiments. The obtainedcomplete equation of state can describe well the high pressure responseof polystyrenein pressure range of 500GPa. The refractive indexandpolarizabilityof polystyrene under shock compression was investigated, and a relationship between the refractive index and the density, pressure and temperature of the polystyrene obtained. Above a pressure of 20GPa, an obvious inflexion was observed in how the refractive index of the polystyrene varied with the thermodynamic variables; in particular it was found to depend strongly on the temperature. Relating the measured refractive index to the polarizability indicates that the polarizabilitydecreasd from 1.28×10^-23cm³ at ambient conditions, to 0.98×10^-23cm³ at pressure of 33GPa, indicating the lowest direct band gap of polystyrene becomes < 2eV, similar to that of many semiconductor materials.