The invention discloses a method for efficiently separating lignin based on dilute acid preimpregnation. The method comprises the following steps: a, removing impurities from a biomass raw material, crushing, and drying; b, impregnating the biomass raw material with an aqueous dilute acid solution, then lyophilizing, and then performing ball-milling and crushing to obtain ball-milled biomass; andc, performing hydrolytic treatment on the ball-milled biomass obtained in the step b by using composite cellulose to remove carbohydrates from the biomass and obtain lignin in a form of residue, and repeatedly washing the residue to obtain a high-purity enzymolysis residue, namely the lignin. The invention provides an environmentally-friendly and high-efficiency lignin separating method; the environmentally-friendly and high-efficiency lignin separating method can be used for preparing high-purity and structurally intact lignin and is directly applied to structural characterization; a novel method is provided for interpreting the original lignin structure in the biomass, and a theoretical basis and a technical support are also provided for forest tree breeding and further high-value application of the lignin.