Abstract: For the wavelet packet decomposition,the spectrum aliasing will occur,which will lead to the difficulty in extracting the energy spectrum of the composite fault feature of the gearbox. A method based on bypass filter to improve the wavelet packet is proposed to study the composite fault vibration signal of the doubly-fed wind turbine gearbox. Based on the vibration signal of a large number of gearboxes in the wind farm,the traditional wavelet packet and bypass filter are used to improve the wavelet packet to extract the characteristic energy spectrum of the gearbox vibration signal. Experimental results show that the bypass filter is used to improve the wavelet packet’s composite fault vibration signal analysis of the doubly-fed wind turbine gearbox,which can effectively avoid the spectral aliasing of the traditional wavelet packet analysis vibration signal and accurately extract the characteristic energy spectrum of each fault state.