Abstract: An analytic solution of Yb-doped double-clad fiber lasers(DCFL) is presented, and some typical parameters which may influence the slope efficiency of DCFL are discussed based on this analytic solution, such as the reflection of pump power and the signal's scatter loss. It is found that the slope efficiency doesn't always increase with the augment of the reflection of pump power. The change of the reflection will affect the optimal length of the fiber. So it is important to balance between the reflection of pump power and the signal loss. Higher concentration of Yb and lower signal loss will also obviously improve the slope efficiency.