The invention discloses a subsynchronous oscillation inter-harmonic wave extracting method of online adaptive frequency change. The method comprises steps of firstly, acquiring original sampling data of phase current and phase voltage of a circuit in real time in an online manner, or through the original sampling data of the phase current and the phase voltage, calculating instantaneous power of the circuit so as to obtain input data to be calculated and extracted; carrying out filtering on the input data through a narrow-band comb-shaped filter group; acquiring real-time output data of each narrow-band filter; comparing output real-time waveform peak values of the narrow-band filters so as to obtain an extreme point of a broadband amplitude value, wherein the central frequency of the filter corresponding to the extreme point is the main oscillation frequency of the oscillation; and carrying out data fitting on the output data of the filters through zero passage comparison, and determining the precise main oscillation frequency. According to the invention, the algorithm can be applied to an online monitoring device of the subsynchronous oscillation monitoring; and online rapid recognition of oscillation signals is achieved.