Enhancing the decay resistance of Populus tomentosa wood through environmentally friendly methods is crucial for improving its durability and market competitiveness. Lignified twigs (LT), typically unsuitable as timber due to their small diameter, are rich in lignin, which degrades during thermal induction to produce antifungal organic compounds. In this context, the objective of this study was to develop a lignified twig preservative (LTP) by thermal induction from the LT of Actinidia chinensis var. Jinyang. The antifungal activity of LTP under varying thermal conditions was analyzed, along with its chemical composition. Enzyme activity, cell membrane integrity, and respiratory metabolism in fungi treated with LTP were examined to elucidate antifungal mechanisms. Additionally, the decay resistance of LTP-treated wood was evaluated. Results revealed that LTP produced under N2 at 220 °C exhibited robust antifungal activity against Trametes versicolor and Gloeophyllum trabeum, attributed to phenolic compounds such as syringaldehyde, syringone, vanillin, and vanillone. LTP inhibited fungal cellulases, hemicellulases, and ligninases by 30%–60%, disrupted cell membrane functionality, and suppressed respiratory metabolism. Poplar wood treated with LTP demonstrated significantly enhanced decay resistance (mass loss < 10%). This thermal-induced feedback pattern shows great potential for LT in wood preservation.