Abstract: AZ31B magnesium alloy continuous roll cast plate was subjected to double-pass friction stir lap welding with different combined form of counterclockwise and clockwise of mixing head. The transverse section observation, micro-hardness, shear test and fracture scanning analysis were conducted.The results show that all welded joints have holes defect and the nugget zone of joint has higher hardness value.The hardness of the transition zone fluctuates greatly, which is slightly lower than that of the base metal.On the whole, the hardness of retreating side is higher than that of advancing side.When the rotation direction of the double-pass welding mixing heads are both counterclockwise, the shear force of the lap joint is the maximum, which is 6.58 kN.The lap joint is broken at the migration line in the upper plate at the advancing side.The fracture mode is brittle fracture.