Abstract: The process of YAG laser and MIG-arc hybrid welding for 50 aluminium-lithium alloy was investigated based on analysing the characteristics of joint formation in laser-arc hybrid welding. Under the condition of the constant welding position, the effect of main welding parameters on the joint formation, including laser power, travel speed, welding current and the distance between hybrid thermal sources, was discussed. The results indicate that a good synergistic effect is shown if the distance between laser and arc is within 0-6 mm. The laser power plays a main role in increasing of depth and backside width of weld. With the increase of welding speed, the width of the weld decreases rapidly, so dos the penetration. But the depth and width of the weld increase obviously with the promotion of welding current.