In the structural design of aero-engines, weight reduction has always been a key point for researchers. Bio-inspired structural design is a potential way to achieve effective weight reduction while ensuring the overall performance of the structure. Inspired by the microstructure of the animal bone, a biomimetic lightweight design is conducted. The load-bearing capacity of bone-inspired lightweight microstructures under different loading conditions are analyzed through numerical simulation. The results show that for the condition where there is weak load along a certain direction, the rectangular-hole bone-inspired lightweight microstructure is the best biomimetic model. Under unknown load conditions, the square-hole bone-inspired lightweight microstructure has the best load-bearing capacity in the weakest direction. The equivalent material parameters of the typical bone-inspired lightweight microstructure are further calculated. With the square-hole bone-inspired lightweight microstructure, a lightweight design of the intermediate casing is performed. The biomimetic lightweight scheme of the intermediate casing reduces the weight by more than 10% compared with the original structure effectively.