Optical signal to noise ratio (OSNR) is one of the key o ptical performa nce indicators for the efficient operation and maintenance of optical communicat ion systems or optical networks.However,for high efficiency transmission systems or modern d ynamically reconfigurable networks,the conventional linear interpolation techniq ue for measuring the OSNR has become unsuitable due to t he accu mulated noise levels be quite different from channel to channel.So many researchers have attempted to develop new techniques of monitor ing the true value of OSNR in recent years.In this paper,an in-band OSNR m onitoring method based on polarization mode dispersion emulator and polarizer is proposed and analyzed.Theoretical ana lysis proves that this method is independent of both polarization mode dispersion (PMD) and chromatic dispersion (CD),and transparent to both bit rate and modula tion formats.Simulation results show that this method has a measurement range from 9dB to 34dB within accuracy of ±0.5dB,and 1dB polarization dependent loss tolerance with 1dB measured e rror.