To solve the problems of poor fusion tendency and pore defect occurred during the process of laser welding of 5083 aluminum alloy thick plate, a fiber laser is used in the ultra-narrow gap wire filling welding test for 5083 aluminum alloy with the thick of 20 mm. The effects of laser power, welding speed and wire feed rate on poor fusion tendency and pore defect are analyzed. The results show that increasing laser power, decreasing welding speed and decreasing wire feed rate are beneficial to reducing poor fusion tendency. Pore defects can be reduced when we reduce laser power or welding speed, and decrease first and then increase while the wire feed rate is increasing continually. The laser wire filling welding for 5083 aluminum alloy with ultra-narrow gap groove (the depth is 17 mm) is achieved with optimal parameters, i.e., distance between laser and filler wire of +1 mm, welding speed of 0.42 m/min, laser power of 3.8 kW, wire feed rate of 3.5 m/min and defocus distance of +20 mm. The poor fusion is eliminated and the porosity is reduced to 0.25%.