In conventional friction stir welding of magnesium alloys, a strong (0001) texture at the welded zone is inevitable, which deteriorates the joint mechanical properties. A novel method called 'double-sided friction stir welding' solved this problem by weakening the texture. However, the texture-weakening mechanism remains unclear. In this study, the mechanism of the texture weakening in double-sided friction stir welded Mg alloy was systematically analysed. The welding temperature and the material flow mode were revealed to control the (0001) texture randomisation simultaneously. The unique material flow at the mid-thickness layer caused by the double-sided friction stir welding was revealed to match the 'beat phenomenon' for the first time, which was revealed as the dominant mechanism for the (0001) texture randomisation.