1. A method for signal extraction of audio signal, comprising: converting an audio signal into a plurality of frames, wherein the frames are arranged in a chronological order; obtaining frequency domain signal of each of the frames; extracting the spectral data of each of continuous N frames extracted from a current frame to a N th frame in the chronological order by separately using each of the frames as the current frame, wherein extracting the spectral data of each of the frame comprises executing a filtering action on the frequency domain signal to filter out signal values which are less than or equal to an estimated value in the frequency domain signal and maintain signal values which are greater than the estimated value to be as the spectral data of the frame; executing a spectral connectivity determining operation for the N frames, wherein the spectral connectivity determining operation comprises: obtaining a signal block list of each of the N frames based on the spectral data included in each of the N frames, wherein the signal block list records one or more frequency index ranges having signal values greater than an estimated value and each frequency index range recorded in the signal block list represents a respective signal block in the frame; and searching for a spectral connectivity between two adjacent frames according to each of the frequency index ranges recorded in the signal block list of each of the adjacent frames, wherein when a frequency index range of a signal block of the first frame of the adjacent frames is overlapping with the other frequency index range of a signal block of a second frame of the adjacent frames or is connected to the other frequency index range of a signal block of the second frame, the two frequency index ranges have the spectral connectivity is determined; determining the signal blocks included in the adjacent frames and having the spectral connectivity as ideal signals; and determining the signal blocks included in the adjacent frames and not having the spectral connectivity as noises; performing a noise reduction operation on the frequency domain signal according to the ideal signals and the noises; converting the output of the noise reduction operation from frequency domain into time domain by performing an inverse Fourier transform operation, and outputting a de-noised signal as an analog signal.