Abstract: Based on the numerical modeling including stimulated Brillouin scattering and stimulated Raman scattering, the propagation-rate equations of ytterbium-doped double-clad fiber amplifiers are solved. The output characteristics of the high-power backward-pumped ytterbium-doped double-clad fiber amplifiers are studied at different signal linewidths, fiber lengths and fiber-core diameters. The methods of suppressing the stimulated Brillouin scattering and stimulated Raman scattering are also investigated. Calculated results show that the two-scatterings can be suppressed simultaneously by using an ytterbium-doped fiber with the length shorter than 0.68 m and the fiber-core diameter larger than 26 μm, or broadening the signal linewidth to more than 0.04 nm.