Abstract: In order to solve the problems of high flame-retardant addition and deterioration of mechanical properties in the modification of wood-plastic composites (WPCs) by traditional expandable graphite, flame-retardant reinforced wood-plastic composites with a multilayered sandwich structure were prepared by laminated hot pressing process and structure optimization design using poplar wood flour (WF), high-density polyethylene (HDPE), expandable graphite (EG) and nano-silicon dioxide (n-SiO₂) as the main raw materials. And the appropriate characterization and equipment such as cone calorimeter, vertical burner, limiting oxygen index (LOI) tester and mechanical testing machine were used to investigate the effects of single layer, double layer and triple layer sandwich structures on the flame retardant and mechanical properties of wood-plastic composites, respectively. The experimental results show that compared with the control wood-plastic composite (WPC-0), the multilayer structure wood-plastic composites exhibite significant reduction of heat release rate, total heat release, smoke production rate and total smoke production, and remarkable improvement of residue yield in the cone test when the contents of EG in the flame-retardant layer and n-SiO₂ in the reinforcement layer are 10% and 5%, respectively. Among all the multilayer wood-plastic composites, WPC-E3B with a triple layer sandwich structure improves its LOI from 20.8% to 30.6% and passes the UL-94 test with a V-0 rating. Moreover, it also shows better mechanical properties compared with WPC-0, such as a 61.9% increase in impact strength and 16.2% and 13.4% increases in tensile and flexural strength, respectively.