Abstract: Choosing 6061-T6 Al alloy and AZ31B Mg alloy as research objects, temperature field during dissimilar friction stir welding (FSW) process was simulated by the Abaqus software. Effect of welding peak temperature in stir zone on weld surface formation was mainly discussed under tool offset on Mg alloy plate. When the welding peak temperature is higher than Al-Mg eutectic temperature, adhesion phenomenon appears near the pin root, which is more serious under lower welding speed owing to the increase of peak temperature. With increasing welding speed, crack in weld surface is more easily avoided. At the rotational velocity of 1200r/min and the welding speed of 40mm/min, 6061Al/AZ31BMg dissimilar FSW joint with excellent-formation surface is successfully attained.