Abstract: The oxidation film as marking material was prepared and marked on the surface of test panel to be welded with the method of anodic oxidation, to research the metal flow behavior of friction stir welding under the action of shaft shoulder. The results showed that the marked oxidation film distributed continuously in welding line, which presents “C” shape as a whole. The analysis on flowing process showed that when the marked oxidation film was in the forepart of shaft shoulder, the metal flowed towards lateral of retreating side of welding line; when it was in the rear of shaft shoulder, the metal flowed towards the center of welding line. In addition, the oxidation film presented “C” shape finally due to the velocity contrast on the vertical direction. From distributed localization of oxidation film, the metal under the action of shaft shoulder migrated from advancing side to retreating side, the migration amount of metal depended on the selected welding parameters.