A first-order Polarization Mode Dispersion(PMD) monitoring technique for phase-modulated optical signals was proposed utilizing the cross-phase modulation(XPM) effect between the input signal and the inserted continuous-wave probe in parallel connection. Because of accumulation of PMD, the XPM effect in nonlinear fiber leads to variation of pump phase. This can cause spectrum broadening resulting in variation of pump power. The technique can suppress the influence of PMD in one branch of the two signals in parallel connection. Then the difference of the two pump power at the same band was used to monitor PMD. The simulation shows that the technique can monitor PMD of 40-Gb/s non-return-to-zero differential quadrature phase-shift keying (NRZ-DQPSK) from 0-30 ps. The dynamic range over 3 dB can be used in accurate monitoring compared to existing methods. Study of the effect of signal rate, CD, pump power and filter bandwidth on the new technique was conducted in detail.