The coherently coupled nonlinear Schrdinger(NLS) equation of the propagation of a light pulse in a birefringence nonlinear fiber has been utilized.We have studied modulation instability in polarization-maintaining step-wise decreasing fiber(SWDF) not only in anomalous dispersion regime but also in normal dispersion regime when the angle of the polarized direction and the birefringence axes is arbitrary.The results show that modulation instability can be produced when the angle of the polarized direction and the birefringence axes as well as input power is arbitrary in anomalous dispersion regime.There are both instability region and stability region in normal dispersion regime.The input pulses have different gain spectra at instability region and input power are different obviously.The relation between gain spectra of the modulation instability and the propagation distance and polarized angel has been given.