The invention relates to an automatic nutting mechanism based on a constant magnet. The automatic nutting mechanism comprises a supporting base; a first supporting plate and a second supporting plate are connected to the supporting base through vertical sliding rails, a second motor is arranged on the first supporting plate in a penetrating mode, and an output shaft of the second motor is connected with a nut rotating head capable of attracting a nut. A third motor is arranged on the second supporting plate in a penetrating mode, an output shaft of the third motor is connected with a second gear, the second gear is meshed with a rack, and a first left centering plate, a first right centering plate, a second left centering plate and a second right centering plate are further connected to the second supporting plate, wherein the first left centering plate and the first right centering plate are used for centering the nut, and the second left centering plate and the second right centering plate are used for centering a stud. The rotatable constant magnet is arranged in the nut rotating head, the nut is attracted by the constant magnet through a magnetism conductive piece when the constant magnet is horizontally arranged, and the nut is not attracted by the constant magnet through a non-magnetism-conductive piece when the constant magnet is vertically arranged. The automatic nutting mechanism has the advantages that the height coincidence of the nutting moment can be guaranteed, the automatic nutting mechanism can be automatically separated from the nut after nutting is carried, and the automatic nutting mechanism is suitable for various nuts in different types.