The invention provides a method for preparing a polyurethane adhesive for wood. The steps are as follows: (1) Grind mulberry fiber and sieve it, dry it, add it to a sodium hydroxide solution, stir it in a water bath, filter it after discharge, and dry it to a constant weight , to obtain alkalized mulberry fiber; (2) adding mica iron oxide powder and alkalized mulberry fiber to ethanol to obtain a mixed solution, adding KH-570 to the mixed solution after hydrolysis, magnetic stirring, and drying after suction filtration to obtain Compound; (3) 50-65 parts of polyurethane prepolymer, 1-2 parts of defoamer, 2-3 parts of solvent are added to the stirring tank for stirring, and after heating, continue to stir, and 6-8 parts of plasticizer, 15- Add 20 parts of filler, 0.5-1 part of coupling agent and 10-15 parts of compound into the stirring tank, continue stirring after heating, lower the temperature, add 0.5-1 part of catalyst into the stirring tank, continue stirring for 40 minutes, and obtain wood after discharging Polyurethane adhesive. The polyurethane adhesive prepared by the invention has good yellowing and aging resistance, bonding strength, heat resistance and environmental protection performance.