The Dougong of "Tiangong Pavilion" in Gongshu Hall, Qi Village, Hu County, Shaanxi Province is studied, which was made in the Ming Dynasty. Fourier transform infrared attenuated total reflection(FTIR-ATR) is used to study the changes of chemical composition and structure of the minor carpentry work. The retention rates of hemicellulose, cellulose and lignin in the minor carpentry work are calculated according to Lambert-Beer Law，and the decay degree of wood components is studied. The results show that FTIR-ATR method is accurate, rapid, nondestructive and reproducible in the quantitative analysis of the main components in wood. Due to long-term exposure to air, the wood components are affected by humidity changes, microorganisms, ultraviolet rays and other factors in the environment. The hemicellulose which is filled and bonded in the wood has been basically completely degraded. The strength, toughness and bearing capacity of wood components are reduced, so it is necessary to strengthen the wood components. The results can provide robust data support for future protection and restoration of cultural relics.