The invention relates to the field of wood material treatment industry, and discloses a method for prolonging the outdoor service life of fast-growing wood. By adopting a way that the fast-growing wood is compounded with an inorganic material, the prepared and obtained inorganic material can exist in a capillary tube of the wood in a physical filling way, the inorganic material can be in bonding with cellulose and can be further in firm combination with groups such as a hydroxyl group and a carboxyl group, and a skeleton of a cell wall can be formed, so that the mechanical performance, the size stability, the flame resistance and the corrosion resistance of the inorganic material are improved, a composite material with extremely high comprehensive performance is formed, and outdoor durability of the fast-growing wood is improved. According to the method disclosed by the invention, a new direction is provided for development of the fast-growing wood, the fast-growing wood is facilitatedto develop from low class to high class and from single function to multiple functions, and high-value utilization is realized; a wooden composite material with excellent performance is prepared, a preparation process is simple in operation and high in efficiency, the method has the advantages of simpleness in operation, safety and reliability, long maintaining life and the like, and the technical scheme is extremely worth popularizing and using.