There are several similar defects of high axial vibration happened during the operation of main feed pump motors in a nuclear power plants， which affect the safety of the equipment. Combined with the structural characteristics of this type of motor， a simple rotors dynamic model supported by spherical bearing on both sides is established. When the self-aligning function of the ball bearing is not good， a mixed air gap eccentricity on both sides of the rotor will appear， and the center line of the journal bearing force will not coincide with the center line of the bearing seat and so on. Under the coupling action of unbalanced magnetic pull and unbalanced centrifugal force， axial vibration similar to radial vibration will be excited. When the mixed air gap eccentricity is dominated by static eccentricity， the frequency spectrum of axial vibration is dominated by twice the rotor rotation frequency， and the amplitude is proportional to the equivalent eccentricity； the amplitude of radial vibration and axial vibration can be simultaneously decreased by the new method of improving the rotor dynamic balance accuracy and reducing the equivalent eccentricity. By improving the working state of ball bearing， optimizing the air gap out of tolerance and balance test of rotor， this common defect is thoroughly dealt with， and the correctness of theoretical analysis is verified.