Maraging steel is widely used in aerospace applications such as aerospace structural steels and rocket engine casings, due to its ultrahigh strength and excellent comprehensive mechanical properties. The materials can be strengthened by precipitating nanoscale phases through aging treatment at moderate temperatures to achieve ultrahigh strength, high ductility and high toughness. Different cobalt contents have a great influence on the aging microstructure of maraging steels. However, the mechanism of cobalt in the microstructure of materials after aging treatment is still controversial at present, and there are fewer studies in this regard. In this paper, maraging steels with compositions ranging from 6% to 12%(mass fraction) cobalt content were designed and prepared, and the aging microstructures were tested and characterized. The results show that with increasing cobalt content, the martensite lath width of the aging microstructure decreases, the dislocation density increases, and the nano-precipitated phase in the material is gradually transformed from the Mo-rich phase to the Ni 3 Ti phase.