The invention discloses a method for preparing cellulose nano-fibers based on a biomass material. The method comprises the following steps: carrying out drying and fining treatment on the biomass material; then, mixing the generated biomass material and an organic acid solvent, heating, reflowing and separating biomass cellulose; then carrying out nano fibration treatment on the separated biomass cellulose by adopting a physical method, so as to obtain the cellulose nano-fibers with high purity and a high length-diameter ratio. By adopting the method disclosed by the invention, the preparation efficiency is effectively improved and the cellulose nano-fibers with the high purity and the high length-diameter ratio can be prepared; the generation of chemical pollution wastes is reduced; meanwhile, a series of environment problems caused by the fact that chlorine is used in a traditional purification method are avoided.