Abstract: To optimize the fiber laser cavity, an Yb3+-doped double-cladding photonic crystal fiber (PCF) was pumped with a 110W multimode LD at central wavelength of 976nm, and the PCF laser characteristics were examined. A dichroic flat mirror and a dichroic flat-concave mirror were used as the back cavity-mirror respectively. There happened the model competition phenomenon in the first laser. And the line width of the PCF laser with a flat-concave mirror as the back cavity-mirror was about 5nm, while another one was above 10nm. Besides, slope efficiency of the former was 11%, and the latter's was just 9%. It has stable model, narrower emission line width and higher slope efficiency in the PCF-laser with a flat-concave mirror as the back cavity-mirror.