The drivetrains of many utility-scale wind turbines have a gearbox connected with a doubly fed induction generator (DFIG). Since gearbox failure is a major contributor to the high maintenance cost of wind turbines faced by the wind industry, it is important to perform fault diagnosis for gearboxes. Among different gearbox fault diagnosis methods, those using current signals collected from generator terminals have shown their merits in terms of complexity, cost, and reliability. In this paper, a new method that uses a DFIG stator current signal for the fault diagnosis of wind turbine drivetrain gearbox in nonstationary conditions is proposed. In the proposed method, the dc offset and high frequency noise of the current signal are first eliminated. Then, the envelope of the current signal is obtained by using the Hilbert transform. The current envelope signal only contains nonstationary frequencies that are proportional to the DFIG shaft rotating frequency. Next, a synchronous resampling algorithm is designed to convert the current envelope signal with a constant time interval to a resampled current envelope signal with a constant phase angle interval. Finally, the power spectral density analysis is used to obtain the frequency spectrum of the resampled current envelope signal from which the constant characteristic frequencies can be easily identified for the purpose of fault diagnosis. Laboratory test data collected from a 1/3 hp DFIG wind turbine drivetrain test rig and field data obtained from three 1.6-MW DFIG wind turbines are used to show the superiority of the proposed method.