Abstract: Different from a traditional double clad fiber laser, the front end of the double clad fiber is separated from the dichroic mirror which forms a Fabry-Perot filter. The efficiency of Fabry-Perot filter is analyzed theoretically. By adjusting the distance between the mirror and the end of the fiber, a novel Yb3+ doped double clad fiber laser with narrow line width and single mode is presented. In the experiment, the laser with an output power of 0.62 W and a slope efficiency of 20% is obtained. The line width at central wavelength of 1 082.79 nm is only 0.14 nm.