In view of the problem of noise interference and frequency ambiguity of vibration signal of wind turbine gearbox under time-varying conditions， the method of tacho-less order tracking for wind turbine gearbox based on time-varying conditions and VMD-SET is proposed， which is based on the exploration of the relationship between vibration signal of wind turbine gearbox and rotation frequency fluctuation law in the circumstance of tacho-less. In this method， firstly time-frequency analysis to vibration signal of gearbox is processed using the method of SET （Synchroextracting Transform） after filtering by VMD （variational mode decomposition）. And fault characteristic frequency trend is extracted preliminarily from bearing fault time domain vibration signal and time-frequency ridge lines of meshing frequency are also obtained from normal gear meshing modulation time domain vibration signal. By fine timefrequency ridge lines cross decoupling， local instantaneous frequency extraction effect is optimized. Then the extracted speed curve is used to track the order of the bearing fault vibration signal， and a single spectral line of the fault characteristic order is obtained from the order spectrum of the angular domain signal after order tracking. Simulations and experiments verify the superiority and effectiveness of the method proposed.