The invention discloses a formaldehyde-free adhesive and a preparation method thereof. The formaldehyde-free adhesive is prepared from the following components in parts by weight: 130 to 150 parts ofwater, 50 to 80 parts of defatted soybean flour, 5 to 15 parts of a toughening agent, 2 to 4 parts of a curing agent, 2 to 4 parts of an accelerating agent, 3 to 4 parts of a modifying agent, 5 to 8 parts of a cross-linking agent, 5 to 8 parts of a diluent, 3 to 4 parts of a mildew preventive, 2 to 5 parts of a filling agent and 1 to 2 parts of a coupling agent. According to the formaldehyde-freeadhesive and the preparation method of the formaldehyde-free adhesive, disclosed by the invention, the technology is simple, no formaldehyde is released, the bonding strength is high, and the formaldehyde-free adhesive is suitable for being applied to a modern indoor home decoration environment.