<p>To improve flame retardancy and reduce toxic gas emissions in polyolefin composites, it is essential to simultaneously regulate oxidation reactions in the gas phase and carbonization behavior in the condensed phase. In this study, spinel high-entropy oxides (HEOs) composed of Fe, Mn, Cu, Co, Ni, and Cr are incorporated into a flame-retardant polyethylene system as multifunctional nanocatalysts to elucidate their flame-retardant mechanisms. Density functional theory (DFT) calculations reveal that typical pyrolysis products such as CO and C₂H₄ can stably adsorb on the HEO surface, prolonging their residence time and promoting catalytic transformation. CO binds to transition-metal sites through its terminal carbon atom, thereby facilitating catalytic oxidation to CO₂. The surface interaction of C₂H₄ further promotes the formation of aromatic and graphitic structures during carbonization. These results demonstrate that HEOs enhance polymer carbonization and structural ordering while markedly reducing smoke and toxic gas release. Moreover, the nanoscale dispersion and catalytic interfaces of HEOs improve the mechanical integrity of the composite. Overall, the HEOs act synergistically in catalyzing gas-phase purification and condensed-phase carbonization, providing a promising strategy for developing efficient flame-retardant polymer composites with enhanced structural performance.</p>