The butt joint of A7204P-T4 aluminum alloy sheets with a thickness of 6 mm was welded via fiber laser-cold metal transition (CMT) arc hybrid welding, with various sizes of niobium foil attached to the butt interface before welding (niobium mass fractions were 0.74% and 1.36% in the weld). Well-formed joints were obtained by optimizing the process parameters. The effects of niobium contents on microstructure and mechanical properties of hybrid welded joints were studied, and the tensile fracture mechanism and fracture morphology of welded joints were also analyzed. Experimental results show that the microstructure of niobium-free welds mainly comprises fine crystal regions, columnar crystalline regions, and equiaxed dendrite regions. After adding niobium foil, grain refinement was observed in the weld metal due to the segregation of niobium solute and heterogeneous nucleation of niobium precipitates, and the apparent disappearance of columnar and dendritic structures were also obserbed. After mass fraction of 0.74% niobium was added, the average grain sizes of fusion zone and weld center decreased by 57.9% and 55%, respectively. The average tensile strength of the joints without niobium, 0.74% and 1.36% mass fraction of niobium were 325 MPa, 334.5 MPa and 328 MPa. The post-break elongation of the welded joint with a mass fraction of 0.74% niobium was 6%, increasing by 71% compared to joint without niobium addition. The fracture surfaces of the three tensile test samples were mainly dimples accompanied by obvious tearing edges, showing microporous aggregate fracture characteristics.