Abstract: Aiming at the problem of poor formation of weld in 30 kW fiber laser welding of aluminum alloy,the images of weld pool and keyhole in the welding process were taken by high-speed camera,and effect of different defocusing amount on keyhole behavior was studied comparatively. The results showed that defocusing amount had a great influence on keyhole and molten pool behavior. In the case of positive defocusing amount,the fluctuation degree of keyhole opening area was more violent,and the back wall of keyhole was easy to form upward flowing liquid column,accompanied by a large number of splashes. The large-scale mutation of keyhole opening area and the falling process of liquid column would greatly disturb the stable flow of liquid molten pool,resulting in defects such as depression and undercut on the weld surface. In the case of negative defocusing amount,the area of keyhole opening was relatively small in both fluctuation amplitude and duration of abrupt change. In the case of-20 mm defocusing amount,the duration of sudden expansion contraction of keyhole was only 1/3 of that of positive defocusing amount,the tendency of liquid column formed by liquid metal upwelling was also small,the welding process was more stable,and the quality of weld surface forming was significantly improved. The penetration of single laser weld reached 29 mm.