Abstract: In order to investigate the existent forms of dyestuff and the combinative forms between dyestuff and wood molecules when wood is dyed, Populus tomentosa stained with two different kinds of acid dyestuffs by vacuum-immerser was studied,and the infrared spectra of wood and dyed wood were analyzed by FTIR. The results showed that neither new absorption peak nor new organic functions regiment appeared during dyeing P. tomentosa with two kinds of dyestuffs but without additive.The absorption intensity of some organic functioning regiments had changed because of the appearance of physical and chemical changes. Partial organic functioning regiments of dyestuff molecule took the place of those of wood and overlay phenomena also existed.