In this paper,the influence of defocus on the process of laser cut-wire filler welding of thick plate 6005 A aluminum alloy was studied by observing and recording the molten pool fluctuation with a high-speed camera. The results show that when the defocus is zero,the spot is small,the energy density and the impact force are large,and the violently reactive liquid wire prevents the stable formation of keyhole in the weld pool,which makes the whole welding process unstable,accompanied by a lot of spatter and the appearance of periodic fine hole. Both positive defocusing and negative defocusing can increase the spot diameter and improve the stability of welding process,and the positive defocusing effect is better than negative defocusing,so it is better to use positive defocusing in laser wire filler welding. For aluminum alloy 6005A with a thickness of 16 mm,the welding speed is 6 mm/min,the defo-cusing capacity is +5 mm,and the laser power is 6 kW. The hardness value of the weld center of the welded joint is the lowest,which is 54 HV. Due to the large number of pores in the weld center,the tensile strength of the welded joint is 173.25 MPa.