Based　on　rate　equations,　this　paper　gives　expressions　of　the　output　power,　the　pump　power　threshold,　the　slope　efficiency,　the　fibre　optimal　length　and　the　laser　gain　of　a　quasi-three-level　Yb-doped　fibre　laser.　The　gain　relationship　between　quasi-three-level　system　and　four-level　system　is　also　given,　which　provides　a　theoretical　basis　for　suppressing　the　four-level　transmissions.　In　the　experiment,　we　adopt　the　backward　pumping　configuration　and　use　a　linear　cavity.　The　Yb-doped　fibre　laser　has　an　output　power　of　372　mW　of　single-mode　laser　emission　at　980　nm　with　a　slope　efficiency　of　21.2%　when　the　fibre　length　is　29.5　cm　close　to　an　optimal　fibre　length　in　theory.　The　experimental　results　accord　well　with　the　theoretical　analyses.