Based　on　rate　equations,　this　paper　gives　expressions　of　the　output　power,　the　pump　power　threshold,　the　slope　efficiency,　and　the　fiber　optimal　length　of　the　quasi-three-level　Yb-doped　fiber　laser.　The　laser　gain　relationship　between　quasi-three-level　mode　and　four-level　mode　in　Yb-doped　fiber　laser　is　also　given,　which　provides　a　theoretical　basis　for　suppressing　the　four-level　transmissions　and　determining　the　range　of　the　fiber　length.　In　the　experiment,　a　plano-plano　cavity　consisting　of　two　0　fiber　ends　is　adopted　as　cavity　mirrors.　A　quasi-three-level　Yb-doped　single-mode　fiber　laser　totally　emitted　1.32　W　of　single-mode　laser　output　at　980　nm　from　two　ends　with　the　slope　efficiency　of　75.3%　when　the　fiber　length　is　36.5　cm.　The　corresponding　optical　conversion　efficiency　is　66%　from　incidence　pump　power　at　946　nm　to　the　laser　power　at　980　nm.　Besides,　a　total　power　of　15　mW　at　490.8　nm　is　obtained　by　frequency　doubling　the　laser　at　980　nm　with　BIBO　with　the　conversion　efficiency　of　1.1%.