The invention discloses a method for preparing a composite biomass adhesive by using papermaking black liquor and belongs to the technical field of adhesives. The method comprises the steps of firstly pretreating the papermaking black liquor so as to remove large-grained impurities from the papermaking black liquor, then, mixing the papermaking black liquor with glacial acetic acid, polyacrylamide and the like so as to obtain a lignin-containing mixture, mixing the lignin-containing mixture with ammonia water, dimethylformamide and the like so as to obtain a modified lignin mixture, stirring the modified lignin mixture with starch, putting the mixture into a vessel, adding substances such as ammonium persulfate and N,N-dimethylformamide into the vessel, and carrying out reaction, thereby obtaining the composite biomass adhesive. Proven by examples, sheet material finished products made by using the adhesive prepared by the method are environment-friendly and safe, the content of free formaldehyde measured finally is decreased to 0.8-1.2% from 5-7%, and the removal ratio reaches up to 84% or more; and the papermaking black liquor serves as a raw material, and the waste is utilized, so that the cost is reduced, the environment is protected, and all indexes detected reach national standards.