A Lewis acid has been considered as a char promoter during the combustion of polymers. The acidified 4A zeolites (HA zeolite) containing Lewis acid sites (4A-L zeolite) and Bronsted-acid sites (4A-H+ zeolite) were prepared and their crystallinity and acidic site structure were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The thermal behavior and flammability of a polystyrene (PS) composite containing different types of zeolite were investigated by thermogravimetric analysis (TGA), volatile gas and condensed phase FTIR analysis, limiting oxygen index (LOI), and cone calorimeter. Both 4A-L and 4A-H+ zeolite could enhance the flame retardancy of a PS composite significantly. The heat release rate (HRR) of a PS composite could be decreased by 48% after adding 2% 4A-L. It is suggested that Lewis acidic sites in HA zeolite can alter the degradation route and enhance the char formation of a PS composite.