Aluminum alloy has the advantages of high strength, corrosion resistance and good toughness, so it is widely used in aerospace, automotive manufacturing, and other fields. Laser processing has the advantages of controllable energy, high energy density, wide range of material adaptation, no contact, almost no cutting force, clean and environmentally friendly, etc. Moreover, the complex features and micro structure functionalization of components can be solved by laser processing. The oblique incidence of the laser will reduce the processing depth of beam waist from the geometric level. And the resulting space vector effect can introduce new processing mechanisms. The processing result could be improved by adjusting the vector effect with important value and novelty. In this paper, the mechanism and forming surface of 7075 be processed by nanosecond laser with different positive inclination angles under the same basic process parameters. It is shown that laser milling can effectively reduce the surface roughness of 7075. The result shows that the when the laser inclination angle (θ) is in the range of 0°~40°, the surface roughness of the machined surface decreases with the increase of θ, and the roughness can be as small as 0.15 μm. When 40° < θ ≤ 60°, the machined surface roughness increases with the increase of θ, but it is still significantly lower than the original surface roughness. The main reason for this phenomenon is that the projected area, shape, and energy distribution of the laser beam waist in the processing area are affected by the laser beam lead angle. The energy distribution affects the material removal mechanism: cladding, vaporization, or sublimation. And under the positive inclination angle, the processed surface is first irradiated and then processed for the second time by the laser beam. The relative posture of laser beam and workpiece is conducive to the discharge of slag, thus improving the processing quality. And pointed out that within the range of experimental conditions, θ = 40° is the optimal inclination angle, because the processed surface has the least undulation and the highest flatness. Moreover, this research has guiding significance for improving the surface quality of laser processing.