Abstract: The present disclosure provides a distortion-free boundary extension method for online wavelet denoising. The method includes: S1: acquiring a signal segment xn, and performing a distortion-free boundary extension on the signal segment to obtain M+N+L data; S2: decomposing a lifting wavelet of the N data to be denoised into j layers to acquire approximation coefficients and detail coefficients; S3: calculating a threshold of each layer of the lifting wavelet; S4: thresholding the detail coefficients of each layer to obtain estimated values of the detail coefficients; S5: performing wavelet reconstruction by the approximation coefficients and the estimated values of the detail coefficients obtained by thresholding to obtain a reconstructed signal after denoising; and S6: outputting data.