The invention relates to a method for preparing a biomass adhesive from alfalfa meal. The method comprises the following steps of: weighing the alfalfa meal, soaking the alfalfa meal into water, adding ethanol, stirring uniformly and filtering; adjusting the pH of the filtrate, then heating the filtrate, preserving the heat, cooling to room temperature, filtering, and drying filter cakes; weighing the filter cakes, putting the filter cakes into a three-necked bottle filled with distilled water, adding sodium dodecyl benzene sulfonate, heating for a period of time, dripping NaOH, adjusting the pH of the solution to 9, adding poly(phenyl methylene isocyanate), and reacting, thus obtaining the biomass adhesive. By adopting the method, the biomass adhesive with good adhesive property is prepared from the meal raw material of alfalfa with wide planting area, high yield and reproducibility, a deep processing product of the alfalfa with industrialized value is obtained, the additional value is improved, meanwhile, the obtained adhesive does not contain the components such as formaldehyde and aromatic hydrocarbon harmful to human bodies, the cost is low, the process is simple, and the adhesive can be applied in the fields of wood veneers, shaving boards, fiber board and the like.