A flotation method involving the control of the unbalance orientation of a rotor is put forward to improve the measurement precision of mass imbalance of an incomplete spherical superconducting rotor. By adding additional weights within the rotor to change its quantity of the mass unbalance, this method limits the azimuth of the mass unbalance to an accurate measurable range. As the additional weights change, the observed values change simultaneously, and the mass unbalance can be derived by a relevant formula. This method can improve the measured precision without any damage to the rotor or influence on its electromagnetic and superconducting properties. The fluctuation of the measured offset is within ±6 μm, and its Z-axis component is within ±8 μm. Moreover, it can improve the working performance of some types of the devices based on the rotor greatly, and can offer some references for other balanced measurements of the machines without mechanical support for its rotor. The measurement equipment of this method is simple, precise, and its experimental period is short.