Abstract: The thermal degradation of polymer materials is an important step in burning process. The smoke and toxicity in burning relate to variety and quantity of the thermal degradation products. Therefore the study on the thermal degradation mechanism can guide molecule design of flame retardant resins. Methacrylate resin (MAER) is made up of methacrylate terminals in unsaturatd polyester resin as main chain and was methyl methacrylate instead of styrene as crosslinking agent. The thermal degradation mechanism and combustion phenomenon of MAER and general unsaturated polyester resins (GUPR) were studied in this paper. The results show that the aromatic ring compound content (14.9%) of MAER thermal degradation products is less than 74.5% of GUPR. Therefore the smoke generation and toxicity of MAER is lower than GUPR. The MAER filled by alumina trihydrate (ATH) has much lower smoke generation and toxicity than unfilled MAER. When MAER composite filled by ATH is burning, no smoke is observed.