In this letter, the theoretical model and rate equations of Ytterbium-doped double-clad fiber amplifiers are introduced. The output performance of fiber amplifiers is analyzed through numerical simulations considering three main factors: active fiber length, pump power, and dopant concentration. It is found that the output signal power experiences a stable growth and then decreases rapidly as the active fiber length becomes longer. It is also revealed that the dopant concentration shows similar effects as active fiber length on the output signal power of fiber amplifiers.