Abstract: An improved method on the structure of double sideband phase detection in optical fiber communication system with non return to zero data is described for real-time accumulated dispersion monitoring, the theory of this method is deduced and the numerical simulation is presented. The results show that this method achieves quite precise monitoring value without any modification of transmitters. In the 1.55 μm region, the range of accumulated dispersion for 10Gb/s data reaches ±640 ps/nm.