By analyzing the microstructure and mechanical properties of Cu foil after femtosecond laser shock in different absorption layers, the absorption layer with better strengthening effect was selected. Femtosecond lasers were used to impact strengthen the copper foil (Cu-nm) coated with a 100 nm thickness absorption layer and the copper foil (Cu-μm) coated with a 100 μm thickness absorption layer. The microstructure and mechanical properties of the two samples were observed and analyzed by scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, and microhardness tester. After femtosecond laser shock, Cu-nm mainly produced deformation twinning, with 60.9% increase in the twinning ratio, 12.8% increase in the ratio of large-angle grain boundaries, and 10.8% increase in microhardness; Cu-μm mainly underwent dislocation changes, with 16% increase in dislocation density, 9.8% increase in the ratio of small-angle grain boundaries, and 2.2% increase in microhardness. In addition, Cu-nm produces greater residual compressive stress after femtosecond laser shock, which not only neutralizes the residual tensile stress of the base material, but also shows as residual compressive stress, while the residual compressive stress produced by Cu-μm after femtosecond laser shock cannot completely neutralize the residual tensile stress of the base material, and still shows as residual tensile stress. A comparative study showed that the femtosecond laser shock on Cu-nm achieved microstructural twinning, changed the residual stress state and improved the hardness of the copper foil surface.