A high power Yb~ 3+ -doped double-cladding fiber laser with improved F-P laser cavity is studied in experiments.There is no obvious difference in optical-to-optical conversion efficiency of the whole system using this laser cavity in experiment,but there are obvious improvements in the ability of high power laser damage resistance.CW output power of 50 W with the wavelength at 1.06 mm is obtained without any thermo-optical problems,dichroic mirror surface damage or drop of the slope efficiency.The slope efficiency and the optical-to-optical conversion efficiency of laser output are 80.2% with respect to absorbed pump power and 31.9% with respect to total pump power respectively.