The method applies variable differential delay appliance of polarization mode dispersion compensator as variable differential delay line to compensate differential group delay of optical signal. The photoelectric detecting tube which converts optical signal to microwave signal is set in series with bandpass filter within communication optical fibre and the said compensator, the filter is for filtering out the described frequency component to be monitored. The program including particle group majorization algorithm formula is inputted in the computer where peak value power of the band-pass filter is fetched to compare with threshold value. If the peak value power is less than the threshold value, the control signal is generated by the computer to control polarization controller and variable differential delay line to have the threshold value reached for finalizing the compensation automatically.