Abstract: Theoretical analysis and experiment of endpumped Yb3+ doped doubleclad fiber laser are presented in this paper. Based on rate equations, the distribution of pump power and laser power in the fiber, the effect of fiber length and reflectivity of cavity minor on output power are studied. The maximum output laser power can be obtained with the optimum fiber length and fiber end as output coupling. That is, the higher the reflectivity of rearcavity mirror is, the lower the laser output power will be. For short fiber, the output laser power and slope efficiency can be improved when a reflective mirror is set at output end to reflect the pump light. In the experiment, the laser with a central wavelength of 1088.3nm, the slope efficiency of 33.7% and the maximal output power of 1.75W were obtained.