The Chengdu Commercial Street boatcoffin burial site is a major archaeological relic of the ancient Shu and also the largest boatcoffin burial site ever found. In order to effectively protect unearthed coffins, it is neccsary to scientifically anlysis the preservation status of the coffins. Elemental analysis, Fourier transform infrared spectroscopy and Xray diffraction were used to analyze changes in chemical composition. The results show that the unearthed coffins have high carbon and nitrogen but low oxygen and hydrogen contents. The coffins have almost completely lost their hemicellulose and have a low cellulose content. The relative content of lignin has increased. The macromolecular chains of cellulose have degraded and the cellulose crystallinity has been destroyed. Therefore, it is necessary to reinforce these coffins to avoid shrinkage, cracking and deformation that may occur during the dehydration process.