The coherently coupled nonlinear Schrdinger(NLS)equation of the propagation of a light pulse in a fiber has been utilized.We have studied modulation instability of light pulse with near frequency not only in anomalous dispersion regime but also in normal dispersion regime in a nonlinear birefringent step-wise decreasing fibers(SWDF),moreover,the polarized direction along the two birefringence axes are orthogonally and the intensities along the two birefringence axes are equal.The results show that modulation instability can be produced both in anomalous dispersion regime and in normal dispersion regime.The input pulses have different gain spectra obviousiy when input power regime is different in normal dispersion regime.Furthmore,the shape of gain spectra is invariable as function of propagation distance when power of input pulse and wave length difference is const.