A coupling system for end-pumping double-clad fibers, formed by a collimating and a focusing lens, is presented. By analyzing the Gaussian beam output from multi-mode pigtail fiber propagation behavior, the parameters of the collimating and focusing lenses and their position relationship are designed for the mode-field matching condition between the pump beam exited from the pigtail of the LD and the guided mode in the inner clad of the double-clad fiber. With a section of undoped double-clad fiber, which has the same specifications as the doped double-clad fiber, the coupled power of 34.1 W is measured experimentally while the pump power output from LD of 50 W, and thus the coupling efficiency of the designed end-pumping coupling system is nearly to be 70%, which provides the foundation for double-clad fiber lasers.