The coupled-mode equation for the magneto-optic four-wave mixing existing in a fiber with magneto-optic and nonlinear effects as perturbation is derived. The parametric processes of the isotropic magneto-optical nonlinear fiber are analytically investigated, and the phase matching condition can be achieved by the dispersion characteristics for the magneto-optic coupling coefficient. In the case that the wavelength dependency of fiber Verdet constant is negligible, the magnetic control characteristics of fiber parametric gain are analyzed for the incident left-hand circular polarization. The necessity of using magneto-optic and nonlinear fiber with high Verdet constant in the experiments is pointed out. The results show that, 1) for the magneto-optical nonlinear fiber with low birefringence, the maximal parametric gain can be obtained by optimizing the magnitude of fiber birefringence; 2) from the monotonic dependence of parametric gain on the magneto-optic coupling coefficient, the magnetic adjustability of parametric gain can be improved by appropriately choosing fiber length, pump power and polarization states of guided optical waves.