The expression of gain is derived from nonlinear Schrödinger equation with consideration of raman effect when two laser pulses with different wavelengths are emitted into birefringence fiber along two polarization axes. The gain characteristic input laser pulses of different frequencies are revealed by comparing the laser pulses of identical frequency. The result show that the gain spectra will apparently differ when input laser pulses have different frequencies in normal dispersion regime and anomalous dispersion regime. The intensities of outboard gain peak about stokes part and anti-stokes part increase and frequency deviates from central frequency as group velocity mismatch increases. Therefore, the THz pulse can be extracted. The gain spectrum will be apparently different from that with no Raman effect taken into account, when the two polarization modes, are in different dispersion regimes. The symmetry of gain spectrum will be damaged. The intensity of gain peak of stokes is apparently stronger than the peak of anti-stoke.