Abstract: In order to solve the problem of the low fatigue strength of single-sided welding (SSFSW) caused by the uneven heat input, double-sided friction stir welding (DSFSW) was used to weld AZ31 magnesium alloy with a thickness of 10 mm. The results show that the yield strength of DSFSW joint is 130 MPa, which is 5% higher than that of the SSFSW joint. The fatigue limit of the DSFSW joint is 88 MPa, which is 76% higher than that of the SSFSW joint. The fatigue crack of DSFSW joint starts at the advancing side of the upper/lower side, crossing the interface of the upper/lower weld seam and breaking in the retreating side (RS) of the one weld seam finally. The fatigue fracture surface of DSFSW is brittle fracture mainly composed of cleavage. The strain range of one side of DSFSW joint is close to the RS strain of SSFSW. The fatigue strength and life of magnesium alloy joint welded by DSFSW have been greatly increased.