A displacement estimation method is disclosed which includes: a signal capturing step for capturing a signal; a displacement estimation step for calculating a displacement based on a state change of the signal; and an output step for outputting the displacement, in which the displacement estimation step includes one or more of the steps of (A) consistency check to determine whether the state change of the signal is consistent with the waveform of the signal; (B) period prediction and compensation to count in the present period according to a proportional length of a previous period; and (C) adaptive threshold update to adjust the definition of the state change corresponding to the amplitude of the signal.