The invention discloses a formaldehyde-free lap adhesive having sweet aroma and a preparation method thereof. The acetylation-modified nano crystalline cellulose is taken as an adsorption material, through acetylation-modified treatment, dispersibility of the nano crystalline cellulose in the lap adhesive is improved, a lot of active hydroxyl groups and high specific surface area can be obtained,and the active hydroxyl groups and a stable covalent bond generated by a chemical reaction with free formaldehyde molecules are connected for fixing most free formaldehyde molecules, so that the freeformaldehyde release amount of the lap adhesive is obviously reduced; the hydrophobicity of the nano crystalline cellulose through acetylation modification is improved, the prepared lap adhesive is used for overlapping cigarette paper, the lap adhesive has sweet aroma, can improve the flavor of the cigarette, can reduce the generation of toxic exhaust gas, keeps tobacco shred quality, keeps circularity of tobacco, has the advantages of good bonding property and strong water proofness, and can avoid phenomena of broken seams, strips, and wrinkles while lapping by a cigarette maker.