The invention relates to an automatic folding fastening system, in particular to an automatic fin folding device. The automatic fin folding device comprises an installation base, a fin angle positioning unit, a fin stripping follow-up unit, a fin folding driving unit, a fastening tool grabbing unit, a folded fin anti-deformation mechanism and an external shaft, wherein the fin angle positioning unit, the fin stripping follow-up unit and the fin folding driving unit are arranged on the installation base. The fin angle positioning unit is used for positioning the angle of a fin on a missile body. The fin stripping follow-up unit is used for positioning the fin and following the fin to move. The fin folding driving unit is used for driving the fin to be folded. The fastening tool grabbing unit and the folded fin anti-deformation mechanism are arranged on the external shaft. The folded fin anti-deformation mechanism is used for holding the folded fin. The fastening tool grabbing unit is used for grabbing a fastening tool. By means of the automatic fin folding device, the folding angle precision of the fin can be ensured, the uniformness of stress applied to the fin in the folding process is ensured, and an oil paint layer of the fin is prevented from being damaged in the folding process.