Depending upon the principle of injection locking, using a DFB LD possessed stable mean frequency and narrow linewidth as a mester laser and cooperating a fiber ring laser as a slave one,when the injection locking excited, the linewidth of the master laser is squashed narrower by better resonating characteristic due to long fiber ring cavity. The stable mean frequency and much narrower linewith laser is gained, and its linewidth has been suppressed down to 70 kHz.