Polarization-dependent gain (PDG) and state of polarization (SOP) of the output signal light in Yb^(3+-) doped fiber amplifiers are studied by numerically stimulation, which showed that PDG of output signal light is not only changed with the input pump power and signal power, input polarization of pump and signal light, but also changed with the doping concentration, the cross-section anisotropy, the fiber length, the phase difference of the fiber, and so on. Moreover, SOP of the output signal light is studied. It is found that the polarization of output signal light is relative not only to the phase difference of the fiber, polarization of pump and signal light as the non-doped fiber, but also to PDG of output signal light, the cross-section anisotropy, the doping concentration, and so on, which is different to the non-doped fiber.