Abstract: The Pinus massoniana inside and outside the Qing Dynasty wooden building from approximately 170 years ago was taken as the research object to systematically analyzes the effects and regulations of natural aging on the microstructure, combustion behavior and mechanical properties of the Pinus massoniana . The results show that natural aging can significantly damage the cellular structure of the pine, where indoor pine is susceptible to significant surface wormholes due to insect infestation and fungal parasitism during service, while outdoor pine is susceptible to more significant degradation of internal organic constituents due to UV light. Compared to the degradation of internal organic components, the surface wormholes can deteriorate the mechanical properties of the pine to a greater extent. The tensile and bending strengths of outdoor Qing Dynasty pine are lower than those of modern pine by 34.1% and 34.7%, respectively, while those of indoor Qing Dynasty pine are reduced by 48.2% and 31.6%, respectively. Combustion performance analysis reveals that the natural aging effect weakens the combustion strength and thermal stability of the wood, with the peak heat release rate （PHRR） being reduced by 8.4% and 15.4%, and the initial decomposition temperature （T5%） being reduced by 19.2 and 10.1 ℃, for the indoor Qing Dynasty pine and outdoor Qing Dynasty pine, respectively, compared to the modern pine. In addition, compared with the outdoor Qing Dynasty pine, the indoor Qing Dynasty pine has lower T5% and higher CO2 and CO production rate, which is mainly related to the greater destruction of the cellular structure of indoor Qing Dynasty pine, thereby indicating that the indoor Qing Dynasty pine has lower thermal stability and stronger fire smoke toxicity.

Key words: Pinus massoniana, natural aging, microstructure, combustion behavior, mechanical properties