For protecting the transmission performance of the Polarization Multiplexed-Quadrature Phase Shift Keying (PM-QPSK) optical transmission system, an improved Optical-Signal-Noise-Ratio(OSNR) monitoring method based on high order statistical moment was proposed. By using different calibrations, this method is transparent for modulation formats. Using numerical simulation the proposed method is verified in 100 Gb/s PM-QPSK coherent receiving system. In the range of 5~25 dB OSNR, the OSNR measurement error is within 0.5 dB for the modulation formats with different duty cycle, and the system has 2 400 ps/nm of chromatic dispersion tolerance and 62 ps of first-order polarization mode dispersion tolerance for 14 dB OSNR with measurement error in 0.5 dB. The improved monitoring method has such advantages of small monitoring error and high tolerance to chromatic dispersion and first-order polarization mode dispersion.