Abstract: Motor current monitoring system has been widely concerned because of its advantages of non-invasive and low cost. But the traditional spectrum analysis is easily affected by inherent harmonic and installation errors, and the high amplitude of fundamental frequency will weaken fault characteristics. In order to reveal the frequency modulation law of motor current due to decrease of meshing stiffness caused by gear fault, the motor current model that contains the meshing stiffness of faulty gear is established, and the instantaneous frequency expression is derived. Aiming at the shortcomings of traditional time-frequency analysis methods with low accuracy of instantaneous frequency estimation, the instantaneous frequency polarview based on the high-order synchrosqueezing transform (HSST) is proposed to extract gear fault features. This method shows the gearbox fault intuitively by detecting the frequency modulation characteristics synchronized with the meshing period of faulty teeth. Instantaneous frequency polarview avoids the interference of inherent harmonic and fundamental frequency, and has the uniqueness of gear fault characteristics. By analyzing the motor current of RV gearbox experimental bench, the accuracy of the proposed motor current model and the distribution law of fault characteristics are verified, as well as the effectiveness of instantaneous frequency polarview method based on HSST in diagnosing gearbox faults.