When the input optical power is comparable with the saturated power of the semiconductor optical amplifier (SOA),SOA has a large nonlinearity,including self phase modulation (SPM) and cross phase modulation (XPM). SPM makes the signal′s spectrum broaden around the central frequency;XPM produces new frequency parts around the probing light. The different dispersions of the link will change the signal peak power differently,leading to the varied effects of SPM and XPM in SOA. Via effectively setting the central frequency and bandwidth of the filters,the different spectrum changes related to SPM and XPM can be detected simultaneously,in order to monitor the link dispersion,whose range may be over ±30 ps/nm. This technique can be used to monitor the residue chromatic dispersion (CD) and polarization mode dispersion (PMD).