The coherently coup led nonlinear SchrÊdinger (NLS) equation of the propagation of a light pulse in a fibre has been utilized. In the paper, by solving the equation, the p ropagation p roperty of the linearly polarized light in a nonlinear birefringent step-wise decreasing fibre has been studied. The results show that when a linearly polarized wave p ropagates in a single mode fibre, the polarization state may change due to the interaction among linear birefringence, nonlinear effect and chromatic dispersion. Accordingly, cross phase modulation and phase2matched parametric four-photonmixingwill be p roduced. We also found that, the p rocedure occurred not only in the normal dispersion regime but also in abnormal dispersion regime, and it changes according to the distance p ropagated. and, the gain spectra will change with token distance seriesm.