The invention relates to a preparation method of an adhesive for a recombined material. The preparation method comprises the following steps: preparing modified micro-nano cellulose, preparing aqueous polymer, preparing biomass composite emulsion, preparing a main agent, preparing a stress type crosslinking agent and preparing the adhesive, wherein the micro-nano cellulose is prepared by performing acid treatment, low-temperature ultrasonic pretreatment, high-pressure homogenization treatment and surface modification on wood or straw fibers which lignin and hemicelluloses are removed from; and the biomass composite emulsion is prepared by performing vegetable protein modification and active monomer grafting on soybean powder. According to the preparation method provided by the invention, formaldehyde is not added, so high environmentally-friendly property is achieved; the synergistically toughening effect of the biomass composite emulsion and the modified micro-nano cellulose is exerted, so that the prepared adhesive for the recombined material has a good toughening effect. The adhesive prepared by the method is very suitable for adhesion of the recombined material and effectively solves the problems that the existing recombined material has low environmentally-friendly property and low toughness.