The utility model relates to an automatic nut tightening mechanism based on a permanent magnet. The automatic nut tightening mechanism comprises a supporting seat, wherein the supporting seat is connected with a first supporting plate and a second supporting plate by virtue of a vertical slide rail, a second motor is arranged on the first supporting plate in a penetrating manner, and an output shaft of the second motor is connected with a nut rotating head capable of adsorbing a nut; a third motor is arranged on the second supporting plate in a penetrating manner, an output shaft of the third motor is connected with a second gear, the second gear is engaged with a rack, the second supporting plate is also connected with a first left centering plate and a first right centering plate, which are used for centering the nut, as well as a second left centering plate and a second right centering plate, which are used for centering a stud; a rotatable permanent magnet is arranged in the nut rotating head, when the permanent magnet is horizontally arranged, the permanent magnet adsorbs the nut by virtue of a magnetic part, when the permanent magnet is vertical arranged, the permanent magnet does not adsorb the nut by virtue of a nonmagnetic part. The automatic nut tightening mechanism has advantage that the highly consistency of the torque for tightening the nut can be guaranteed, and the automatic nut tightening mechanism can be automatically separated from the nut after the nut is tightened and is suitable for tightening different nuts of different model numbers.