The oligophosphate (OPP) was synthesized by step-reaction polymerization using phenyl dichlorophosphate(PDCP), ethylene glycol(EG), methanol, the composites OPP/PS was prepared by polystyrene as matrix and oligophosphate (OPP). The short chain chemical structure and long-range structure of molecular chain characteristics and relative molecular mass of OPP was explored by FT-IR and gel permeation chromatography(GPC). The mechanical properties, thermal stability and flame-retardant properties of the OPP/PS was evaluated by the SEM, XPS, universal material experiment machine, TGA, limit oxygen index(LOI) and microscale combustion calorimeter. The thermal decomposition activation energy(Ea) of composites were calculated by Flynn-Wall-Ozawa(FWO) method. The results show that the heat release rate peak (PHRR) decreases, the total heat release (THR) decreases, and the LOI increases gradually and the residual carbon rate increases with the increase of OPP content in OPP/PS composites. When the w(OPP)=10%, the PHRR of the OPP/PS composites was 585.00 W/g, the THR was 14.41 kJ/g, the LOI was 24.5%, the residual carbon ratio in the nitrogen atmosphere is 11.27, the Ea decreases with the increase of thermal degradation rate α, thermal decomposition products of OPP polymetaphosphate catalytic substrate into carbon, and covered the surface of a partition the material in the layer of heat and oxygen and carbon layer, to improve the thermal stability and flame retardant properties of OPP/PS composites.