The power-ratio-tunable dual-wavelength fiber laser with linearly-polarized output and its polarization maintaining fiber power amplifier are demonstrated. Two pairs of fiber Bragg gratings with polarization-matching techniques are employed as the mirrors of the resonator to insure the linearly-polarized output with central wavelengths of 1035 nm and 1030 nm, respectively. By introducing the cavity loss at 1035 nm, the power ratio of the two wavelengths can be continuously changed. With the dual-wavelength fiber laser as the seed, two-stage polarization maintaining fiber amplifier is established to scale up the output power. By changing the power-ratio of the two wavelengths of the seed, the affection of seed characteristics on the dual-wavelength power amplification is studied to realize the adjustable power-ratio of the amplified output. When the output amplitude of dual-wavelength on the spectrum is tuned to be equal, the maximum output power of 7.77 W is achieved. Future power scaling operation can be achieved by increasing the pump power or adding the amplification stages, which provides the possibility in application to generate terahertz wave by optical nonlinear difference frequency.