Abstract: In order to solve the problem of weld bead formation and porosity during welding of thick aluminum alloy with 10 kW level normal laser beam, this paper proposed thick aluminum alloy vertical up welding process with 10 kW level oscillated laser beam. 5A06 thick aluminum alloy was applied to conduct the welding experiments, the weld bead formation, microstructure and the mechanical property of the obtained weld bead was analyzed. Results showed that, the process significantly improved the weld bead formation and porosity defects of thick aluminum alloy laser welding. According to the weld pool image, oscillated laser beam stretched the weld pool geometry, therefore the interference of the laser beam on the tail of weld pool could be reduced, which was beneficial for the weld bead formation and porosity. The process successfully welded 24 mm root face of 50 mm thick 5A06 aluminum alloy by double side and double passes at narrow gap groove. When the oscillated amplitude increased, which was parameter II and III, the weld bead was smooth and continuous, and there was no porosity found by X ray inspections. The microstructure by parameter II consisted of equiaxed grains, the microstructure by parameter III consisted of equiaxed grains and few columnar grains. Due to the welding speed was lower with parameter III, the heat input was higher, the grain size with parameter III was larger. The tensile strength with parameter II and III was 337 MPa and 322 MPa, reaching 93.6% and 89.4% of the base metal. The fracture surface was full of dimples, indicating ductile fracture.