Theoretical analysis and calculations indicate that the gain of a semiconductor optical amplifier (SOA) will increase if reflectivity increases appropriately. By selecting appropriate refractive index and thickness of the antireflection film, the reflectivity of TM mode is larger than that of TE mode in a range of wavelength, such a reflectivity distribution can increase the gain of TM mode relatively, improve the polarization insensitivity of SOA and give SOA more flexibility in designing active region and waveguide to meet the requirements needed by other performances.