The high quality joints fabricated by laser arc hybrid welding have certain differences in the crosssectional geometry of the weld seam. In order to study these differences quantitatively, the back width to surface width ratio (R_w) was proposed. Based on the optimization experiments of 7050–T7451 high-strength aluminum alloy with 3 mm thickness by laser arc hybrid welding, three typical welded seams were selected to investigate the correlation effect of different R_w on the macroscopic formation, microstructure, microhardness and tensile properties of the joints. The results show that, only when the Rw exceeds a certain threshold, the weld width on both sides becomes more uniform, and porosity defects are effectively controlled. The microstructure characteristics of the weld zone from the fusion zone to the center of the weld are fine grains layer, columnar dendritic and large-scale equiaxed grains, respectively. With the R_w increasing, the width of the columnar dendritic structure gradually decreases until it disappears, while, the average grain size at the center of the weld seam gradually increases, and the tensile properties of welded joints increase firstly and then decrease. When the R_w is about 0.70, the tensile properties reach the best, with an average ultimate tensile strength of 390 MPa and elongation after fracture of 2.9%. The tensile fractures present a mixed fracture characteristic dominated by brittle fracture.