Transparent copolymers composed of hexa (allyl 4-hydroxybenzoate) cyclotriphosphazene (compound 1) and styrene as potential halogen-free, flame-retardant optical resins were prepared by radical copolymerization. The thermal performances of the cured resins were studied with thermogravimetric analysis; the decomposition mechanism of the copolymers was investigated with integrated thermogravimetry-Fourier transform infrared analysis. Compared with conventional polystyrene, the synthesized copolymers exhibited a higher refractive index and a higher thermal stability both under nitrogen and air atmospheres at elevated temperature, and the visible-light transmittance of the copolymers decreased slightly. With increasing ratio of compound 1 to styrene in the copolymers, the onset decomposition temperature and the char yield both increased gradually.