Abstract: In response to the limited application of wood-plastic composites (WPCs) in structural engineering due to insufficient mechanical performance, dimensional stability, and weather resistance, this study developed and fabricated a novel ternary co-extruded composite material (Co-WPCs@wood). Using co-extrusion molding technology, the material integrates a weather-resistant modified polyethylene (PE) shell, a WPCs intermediate layer, and a high-strength and tough wood core layer to synergistically construct a ternary composite system. A range of Co-WPCs@wood samples were systematically tested for flexural and impact properties, dimensional stability, long-term creep resistance, and UV aging resistance, and were compared with commercially available high-performance Co-WPCs flooring. The results show that the specific flexural strength and specific modulus of Co-WPCs@wood are 2.9—3.4 times and 2.6—3.0 times than those of the commercial Co-WPCs, respectively. The material exhibits significantly enhanced low-velocity drop-weight impact energy absorption and release capabilities, and the modified polyethylene/WPCs composite shell provides excellent protection for the wood core. After multiple cycles of boiling and drying, the water absorption rate of melt-sealed Co-WPCs@wood with modified polyethylene remains below 0.5%, with dimensional changes far less than those of commercial Co-WPCs, demonstrating outstanding dimensional stability. A 1500 h creep test indicated that its creep strain is only 16.0%—30.7% that of Co-WPCs. After 3000 h of accelerated UV aging, the impact strength retention rate of the modified PE/WPCs shell layer reached 98.7%, and the total color change (ΔE*) was only 1.54. This study provides a lightweight, high-strength, weather-resistant, and cost-effective novel composite material technology for load-bearing components in green buildings, thus expanding the prospects for high-end structural applications of WPCs.