Abstract: An electro-optical feed-forward line width compression system based on single side-band modulation was put-forward, which included phase noise extraction as well as single side-band modulation. The phase noise extraction outputted an error voltage signal proportional to the phase error, and the voltage signal modulated the laser signal through the linear frequency modulation's effects on the single side-band intensity modulator. The single side-band modulator, adopting the structure of MZ intensity modulator, equivalent to inline four phase modulators, achieved intensity modulation through phase modulation, and finally got the line width compression laser signal. The experiment aims at DFB semiconductor laser, with wavelength of 1 552.52 nm, power of 10 mW, and through the construction and simulation of the closed-loop system, the laser line width decreases from 0.5 nm to 0.016 nm, which realized the compression of DFB semiconductor lasers.