The invention relates to a method for modifying low-density wood. The method comprises sucking wood, increasing weight, soaking, drying, thermosetting and compressing, wherein the water content of the wood subjected to sucking treatment is 28-32 percent; the gain in weight of wood subjected to weight increasing treatment is 30-35 percent; the gain in weight of the wood subjected to soaking treatment is 15-20 percent; the water content of the wood subjected to drying treatment is 15-18 percent; and a soaking agent comprises the following components in parts by weight: 100 parts of thermosetting resin, 10-20 parts of water-soluble natural organic matter, 3-5 parts of aqueous waterproof agent, 0.5-1 parts of fire retardant and 1-2 parts of surfactant. In the modifying method, a physical modifying technology, a chemical modifying technology and a mechanical modifying technology of hot compression deformation are organically combined, so that modified wood has high density, high intensity and the like of high-level wood, has higher size stability than high-level wood, and has the characteristic of low cost compared with high-level wood.