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%.