Abstract: Presents a new experimental result about an extremely narrow spectral line-width,single mode output obtained from an Er-doped optic fiber laser with photonics injection induced mode-locked.The principle of compressing line-width by mode-locked technique is analyzed.Influence of the instantaneous optic fiber grating due to spatial hole in the saturable absorber on laser line width is discussed.Thus the condition of high side mode suppression rate is obtained.Experimental results demonstrate the laser output to be a stable single mode of line width 30 Hz,having maximum output power of 2 mW,and FSR of 1.875 kHz in multimode.