Abstract: Abstract: Wood is one of the oldest renewable resources used by human activity because of its many excellent material properties, such as good mechanical strength, aesthetic appearance, and easy processing. The main components of wood are cellulose, hemicelluloses, and lignin, accompanied by minor contributions of low molecular weight compounds and mineral salts. The major drawback of wood, intrinsically connected with the structure of its three main macromolecular components, is that it is easily affected by environmental factors, such as light, water, temperature, and biological organisms. Wood modification is a generic term describing the application of chemical, physical, and biological methods to improve its' properties. The aim is to improve the performance of the wood, including improvements in dimensional stability against moisture and bio-deterioration, mechanical property, and weathering resistance. In the case of chemical modification, this involves treatment with various chemical polymers to reduce the content of hydroxyl groups. To enhance the applied quality, the functional wood modifier was used to modify the fast-growing poplar. The stress relaxation and the profile density of fast-growing poplar were analyzed. Moreover, the fast-growing poplar was characterized by the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Scanning Electron Microscope with Energy Dispersive X-Ray Analysis (SEM-EDXA). The results demonstrated that the plasticity of wood enhanced for stress relaxation dropped with increasing amounts of modifier. The XRD analysis showed that the crystalline properties of wood obviously increased, but the ordered structure of the crystalline region on the remaining cellulose was not disrupted after modification. FTIR data confirmed the cross-link reaction between wood fiber and modifier. The XPS analysis results indicated that the content of carbon elements decreased while of oxygen elements increased. The SEM-EDXA showed that the wood modifier dispersed in wood fiber and other vertical cells and the N and C had better interfacial adhesion and dispersion between cell wall, intercellular space and even the nucleus of wood cells. The originality of this paper is in the modification of fast-growing wood using the method of vacuum-pressure-vacuum impregnation with a chemical multi-solution. Moreover, the chemical modification overcomes the uneven distribution of modifier in the wood inner structure. The reaction between wood and modifier is a netted site reaction, depicted as two steps. First, the pre-polymer, urea, and catalyst were sufficiently mixed before impregnation. Then, the fresh, natural wood was soaked in water. It also can be seen that the vessels and cell walls of the wood were open. After the impregnation of the wood cross-section, the wood modifier was evenly distributed in wood fiber and other vertical cells after thermal treatment. The wood modifier reacted with the wood fibers, forming networks and exhibiting higher physical properties and dimensional stability. Further work should be performed to develop the multi-function modifier, and is currently under investigation in our laboratory.