The invention discloses a wood modifier which comprises the following substances in part by weight: 26 parts of sodium carboxymethylcellulose, 3 parts of Arabic gum, 4 parts of octyl-2-cyanoacrylate, 37 parts of ethanol, 1 part of an ammonium polyphosphate-zinc borate composite flame retardant, 0.2 part of citric acid, 19 parts of water, 0.07 part of benzoic acid, 13 parts of polylactic-co-glycolic acid, and 0.03 part of benzalkonium chloride, wherein the pH value of sodium carboxymethylcellulose is 4.5; ammonium polyphosphate is twice of zinc borate in weight, the degree of polymerization of ammonium polyphosphate is 23-25, the phosphorus content of ammonium polyphosphate is 30%, and the nitrogen content of ammonium polyphosphate is 16%; and the molecular weight of polylactic-co-glycolic acid is 6,000-8,000. The wood modifier, prepared according to the preparation method, is high in wood dimensional stability, high in decay resistance and low in water absorption swelling rate.