Abstract: Polystyrene(PS) was modified with hexabromocyclododecane(HBCD) as a flame retardant and then was foamed by using supercritical CO2 as a foaming agent and hydrotalcite(HT) as a thermal stabilizer. The flame-retardant mechanism and foaming behavior of flame-retardant PS were studied. The results indicated that the thermal stability of HBCD could be improved by adding a proper amount of HT. However, the excessive amount of HT reduced the flame retardancy of PS. The thermal stability of HBCD was significantly influenced by the processing technologies and especially by shear flow field and processing temperature. The flame-retardant PS achieved a limiting oxygen index above 30 vol % through foaming with supercritical carbon dioxide.