The modulation on stator current signal of induction motor under the situation of load torque vibration and gap eccentricity is analyzed respectively. In view of that the modulation results in these two cases are difficult to be distinguished in frequency domain while the vibration frequency and rotor frequency are consistent, the Wigner timefrequency domain analysis method is used to calculate the phase difference of sidefrequency current. The work in this paper points out the difference of modulation effects between load torque vibration and gap eccentricity in timefrequency domain, which provides theoretical basis for motor airgap eccentric fault diagnosis.