Abstract: The 1 kW fiber laser of single mode was used to weld the 5052 aluminium alloy with 1 mm thickness. The orthogonal experiment with three element of laser power, welding speed, defocusing amount was designed, and the best process parameter was the power of 900 W, welding speed of 100 mm/s，and the defocusing amount of 2 mm.The results of the tensile tests showed that the highest tensile strength is 282 MPa, achieves to 72% of that of the base metal. The cross section of the joints showed that full-penetration, no blowhole and no lack of materials, the strength of welded joint is the highest.