For a high-speed rotor in a magnetically suspended control moment gyroscope (MSCMG), the gyroscopic effect is one of its prominent characteristics. To improve the control stability of this kind of high-speed rotor in an MSCMG, an extremely important parameter of the rotor named the inertia ratio is defined originally and its effects on the rotor’s gyroscopic effect are analysed theoretically by simplifying the rotor into a standard model during its mechanical design precessing. This research indicates that the inertia ratio of a rotor is determined not only by the rotor’s mass and material density, but also by its generalized stiffness and shape coefficient. For an MSCMG with speed 12,000 rev/min and angular momentum 200 Nms, the optimized range of the rotor’s inertia ratio can be selected as 1.780–1.9, which is more accurate than the experimental one (1.4–2.0). In this case, the precession frequency is reduced by 19.2%, the mass is reduced by 4.63% and angular momentum is increased by 24.45%. This paper will provide helpful hints for the design and optimization of a high-speed rotor.