With the help of supersonic fine particle bombardment technology, a gradient nanostructure with a certain layer depth was constructed on the surface of an Mg-1.8Zn-0.5Zr-1.5Gd biological magnesium alloy. The effects of bombardment time on the microstructure, mechanical properties, and corrosion resistance of the gradient nanostructure were investigated. The results showed that the layer depth and surface roughness of the gradient nanostructures increased with an increase in the bombardment time, and the corresponding mechanical properties and corrosion resistance first increased and then decreased with an extension of the bombardment time. When the bombardment time was 30 s, the alloy had good mechanical properties and corrosion resistance. Its tensile strength, yield strength, elongation, and static corrosion rate were 299.1 ± 2.2 MPa, 264.4 ± 1.5 MPa, 36.8 ± 1.3 %, and 0.307 ± 0.015 mm y −1 , respectively. The results of the 120-h immersion experiment in simulated human body fluids showed that the average corrosion rate of the alloy first decreased, then increased, and finally decreased with an extension of the immersion time and finally tended to stabilize.