The　interference　of　space　beam　in　gain　medium　generates　space　burning　hole　phenomenon,　and　forms　the　gain　grating.　The　gain　grating　formed　by　the　four-wave　mixing　function　has　adaptive,　self-tuning　Q　and　spatial　filtering　capability,　and　can　obtain　holographic　conjugate　output.　But　all　of　them　start　from　the　spontaneous　emission　in　the　cavity.　Spontaneous　emission　is　easily　influenced　by　the　pump,　and　its　bandwidth　is　wide,　which　will　cause　a　certain　fluctuation　in　the　output　frequency.　In　this　paper,　a　single-frequency　Q-switched　pulse　output　locked　in　the　seed　spectrum　range　is　obtained　by　an　external　single-frequency　narrow　linewidth　source　injection　in　a　self-starting　single-frequency　laser　based　on　a　gain　grating.　Comparative　experiment　shows　that　in　the　same　observation　time,　the　spectral　stability　of　the　injected　output　is　much　higher　than　the　self-starting　output.　This　phenomenon　also　proves　that　the　steady　frequency　output　can　be　obtained　by　the　adaptability　of　the　gain　grating　formed　by　external　seed　injection.　©　2018,　Chinese　Lasers　Press.　All　right　reserved.