The invention relates to an automobile hemming adhesive and a preparation method thereof, belonging to the technical field of single-component epoxy adhesives. The automobile hemming adhesive is mainly used for bonding an automobile hemming part. The automobile hemming adhesive is mainly characterized by comprising (A) epoxy resin, (B) modified epoxy resin, (C) toughener, (D) epoxy diluter, (E) thermoplastic resin, (F) plasticizer, (G) thermal stabilizer, (H) latent curing agent, (I) coupling agent, (J) filler and (K) pigment. The preparation method comprises the following steps: (1) evenly stirring (E) and (F) at 5-35 DEG C; (2) adding the mixed liquid composition in the step (1), and (A), (B), (C), (D), (G), (H), (I), (J) and (K) into a movable mixer, and evenly stirring at 5-35 DEG C; (3) passing the mixture in the step (2) through a three-roller machine once or twice; and (4) vacuumizing the movable mixer, stirring, defoaming, and discharging for later use. The invention mainly solves the problems of low environmental-protection index, poor technical properties of sizing, short storage period, low modulus, poor moisture and heat resistance and the like in the existing hemming adhesive.