•Elevated the average power of linearly polarized supercontinuum to over 300 watts, while preserving good beam quality.•A spectral subtraction method is introduced in detail and demonstrate that it can measure the polarization extinction ratio of hundred-watt level linearly-polarized supercontinuum and accurately reflect the polarization state at each wavelength range.•An interesting four-wave mixing phenomenon between the two orthogonal polarization states is observed. We report an all-fiber linearly-polarized supercontinuum laser source with 322.5 W average output power and spectrum covering 800 nm to 2200 nm. A spectral subtraction method is utilized to measure the polarization extinction ratio (PER) of the laser source, which can directly measure the PER of each single wavelength within a wide spectral range. Through detail observation of the PER distribution along wavelengths, an interesting four-wave mixing (FWM) phenomenon between the two orthogonal polarization states is found in the laser system.