High power double-clad fiber lasers under multi-point pumping facilitate the increasing of the power and optimizing of the output performance. The forward and backward pump mode is calculated respectively. The calculated results show that pump power reflectivity at the end of the cavity affects the output performance and the optimal fiber length apparently. The relation of the number of pump point and the output power is obtained from the numerical modeling,furthermore,the optimal fiber length as a function of pump point is obtained. The conclusion can be used as reference for the design of the double-clad fiber laser configuration under multi-point pumping.