Abstract: The Al/Mg composite shell with internal grid ribs combines the lightweight and high-strength properties of magnesium alloys with the corrosion resistance of aluminum alloys. Its grid-rib structure enhances the overall strength of the shell, offering broad application prospects in aerospace and underwater equipment. In this paper, threaded grooves were prefabricated on the inner surface of the aluminum tube, and an internal spinning process was employed to fill the grooves with magnesium tube material. Subsequently, a segmented grid rib core mold was designed, and an external spinning process was used to form a complete grid rib structure on the inner surface of the magnesium tube. The Al/Mg composite shell with grid inner ribs and a three-dimensional mechanical interlocking interface was successfully fabricated. The influence of the forming process on its microstructure and mechanical properties was investigated using Abaqus numerical simulation combined with experiments. The results indicate that under spinning load, the magnesium alloy fills the threaded grooves. The filling rate increases with the thinning rate, tending to saturate when the thinning rate reaches 25%. During the external spinning process, optimal forming quality of the grid inner ribs is achieved at a thinning rate of 40%, and the interface evolves from unidirectional embedding of magnesium tube material into the prefabricated grooves of the aluminum tube to a corrugated mechanical interlocking interface with bidirectional filling between the aluminum and magnesium tubes. After the two-step spinning process, the magnesium side exhibits a fine-coarse-fine grain structure, while the grain size on the aluminum side gradually decreases from the inner to the outer layer. Micro-nano mechanical property tests reveal that the hardness of the inner and outer layers on the magnesium side is higher than that of the middle layer, and the hardness on the aluminum side decreases from the inner to the outer layer. The Al/Mg composite shell with internal grid ribs formed by the two-step spinning process exhibits excellent quality, effectively solving the problems of deformation incompatibility and weak interfacial bonding at the grid rib locations.