According to the wave equation of the optical pulse propagating in the nonlinear fiber, the nonlinear coupled equations is deduced under the Raman effect and the parametric amplification in the low-birefringent fiber. The gain spectra is obtained from the coaction of Raman effect and parametric amplification when the pump linearly polarized along the fast axis of low-birefringent fiber. The results show that the Raman effect can lead to asymmetric of Stokes and anti-Stokes gain spectra. The maximum value and location of the gain spectra are different when power of input pulse and propagation constant are variable.