Abstract: The influences of rotational magnetic field on microstructures of Al-12Si alloy for laser welding have been analyzed.The test results show that the rotational magnetic field can change the behaviors in motion of liquid Al-12Si alloy,refine the weld metal grain and eliminate the drawbacks in Al-12Si weld metal,although laser beam has nonmagnetism and is not conductor,and Al-12Si alloy is nonmagnetic material.For laser welding,induction current of liquid metal in welding pool is caused,and the electromagnetic stirring force on liquid metal with electric charge lead to the rotational motion of liquid metal in welding pool,when the rotational magnetic field passes liquid metal.The higher the rate of the rotational magnetic field is,the larger the electromagnetic stirring force on liquid metal is.