To study the influence of expansion temperature on the properties of expanded graphite (EBG), EBG 300 , EBG 600 , and EBG 900 were prepared by heating expandable graphite (EG) at 300, 600, and 900 °C, respectively. Furthermore, the influence of these EBGs on the combustion performance and physical-mechanical properties of linear low density polyethylene (LLDPE) were investigated. The expansion volumes of EBG 300 , EBG 600 , and EBG 900 increase with the rise of temperature, and a four-stage ordered structure of “graphite worm” gradually forms. The thermal stability increases gradually for EBG 300 , EBG 600 , and EBG 900 . On the contrary, the thermal conductivity decreases in sequence. However, the incorporation of EBG 900 promotes the formation of a continuous network structure and makes the modified LLDPE to present the best heat transmission. The addition of 30 wt% of these EBGs significantly improves LLDPE’s flame retardancy and high-temperature thermal stability. The total heat release, the peak value of heat release rate, and the fire growth index of 70LLDPE/30EBG 300 reduce by 69, 91, and 87% respectively, while the effective fire performance index improves seven times. The addition of these additives reduces the tensile strength and elongation at break, the larger the EBG size, the more obvious the effect.