According to the perturbation theory for magneto-optic (MO) effects and nonlinear effects, the nonlinear coupled-mode equations are derived for optical pulses propagating in magneto-optic nonlinear fibers. The time and frequency domain schemes of magneto-optic effects in the modified split-step Fourier method are brought into comparison, and it is found that both of them are the same, when the step-length is short enough. According to the coupling of magneto-optic effects, nonlinear effects and dispersion, the propagation characteristics of optical pulses in the MO fibers are analyzed in detail. By approporiately adjusting the MO coupling coefficient, the pulse shape can be flexiblely controlled, and the frequency chirp related to the nonlinear effects is also be changed, which is useful for the dynamic reshaping function based on pulse widening. The theoretical analysis results presented in the article may help to develop novel magneto-optic information processing devices with applications to the fields of fiber optic communications and fiber optical sensors etc.