By using phase modulation,single-frequency laser can broaden the spectrum and increase the threshold of stimulated Brillouin scattering of the fiber amplifier,so the output power of fiber amplifier can be improved,but the high coherence of single-frequency laser is also destroyed.According to the principle formula of spectral broadening by phase modulation,the idea of laser line-width compression is proposed in this paper,which can be used to recover the coherence after laser amplification.According to the principle of line-width compression,the construction of line-width compression device is put forward,that is,two phase modulators are used,one is responsible for spectral broadening,the other is responsible for line-width compression after fiber amplification,and the control conditions for realizing phase difference adjustment and amplitude adjustment of line-width compression are put forward.Due to the lack of experimental conditions,this paper only analyzes the theoretical model.