A 5 DOF magnetically suspended flywheel with vernier gimballing capacity which is composed of a conoid reluctive bearing and a Lorentz magnetic bear was investigated and its rotor rim was designed optimally. Based on the structure of the rotor and the goal to minimize the mass of the rim, the mass, inertial moment and resonance frequency of the rim were analyzed theoretically to confirm the optimal variables. Consequently, an optimal design was achieved through iSIGHT and ANSYS , and by taking the number of spokes into account, the variables were optimized by the sequential quadratic programming algorithm in the restrain cases of the resonance frequency, inertial moment, maximum equivalent stress, and the ratio of polar inertia moment to equinoctial inertial moment. Those results of optimization indicate that the mass of the rim is decreased from 2.226 kg to 2.036 kg (namely reduced by 8.54%) when the number of spokes is 3 and other design variables are optimal. The proposed optimal design method can improve the rationality and efficiency of rotor design, and will be an important part in the optimal design of flywheel systems.