The invention discloses a method for preparing an environmentally-friendly adhesive for a lignin-based timber. The method comprises the following steps of: 1) adding phenol, formaldehyde solution, lignin, first alkali liquor and diluting water into a reactor, heating to the temperature of between 75 and 85 DEG C and reacting for 2.5 to 3.5 hours, wherein the molar ratio of formaldehyde to the phenol is 2.20:1-2.40:1; the formaldehyde solution is added for three times in a mass ratio of 1.5 to 1.0 to 1.0; and the reaction time ratio of the added formaldehyde solution is 1.2 to 1.0 to 1.0; 2) cooling to 70 DEG C, adding a formaldehyde collector and second alkali liquor, and reacting at the temperature of between 65 and 70 DEG C for 20 to 50 minutes; and 3) cooling to the temperature of between 30 and 40 DEG C, adding an intensifier, stirring for 10 to 30 minutes, cooling and discharging. In the method, a 50 to 60 percent petroleum-based phenol raw material can be substituted; the methodis stable in reaction, easy to control and easy for industrial production; formaldehyde emission of a plywood reaches E0 level; adhesive strength meets the requirements of national class I plates; and a new approach is provided for the development of an environmentally-friendly biomass-based timber adhesive and recycling of wastes.