The power distributions of signal and amplified spontaneous emission in continuous- wave high-power ytterbium-doped double-clad fiber amplifier are studied. The relationships between the output power and the signal power, pump power, pump schemes, fiber length are studied. The system of ytterbium-doped double-clad fiber amplifier is set up and amplified laser of 48.2 W is obtained when the powers of signal and pump are 2 W and 72.9 W, respectively. The resoults of theoretical study and experiment are in good agreement. And the results of numerical simulation provide a theoretical guide to optimal design of continuous-wave high-power double-clad fiber amplifier.