The high power fiber laser is used to conduct a welding experiment on 3 mm thick aluminum alloy, and the effects of the defocusing amount on the morphology, penetration depth, penetration width, root humps, aspect ratios, and width ratio coefficient of weld pools are systematically investigated. The results show that, with the increase of the defocusing amount, the welding mode is transformed from the deep-penetration welding to heat-conduction one, the weld width does not change obviously, the root hump height and the width ratio coefficient decrease; with the high power fiber laser, the degree of welding bead subsidence and the height of root humps can be reduced by cutting back the negative defocusing amount or transforming into positive defocus; with the low power fiber laser, the back weld width could be increased by strengthening the negative defocusing amount or reducing the positive defocusing amount.