The problem of deducing two-dimensional theory from three-dimensional theory for a soft ferromagnetic elastic isotropic body is investigated. By using the general solution for the soft ferromagnetic elastic solids and Lur'e method, it is shown that the general deformation can be decomposed into two independent parts: the asymmetric deformations (plate problems) and the symmetric part (plane problems). A refined plate theory which takes into account the transverse shear deformation can now be explicitly established without employing assumptions. The exact equations and solutions for the plate with homogeneous boundary conditions are derived, and the equations can be decomposed into four governing differential equations. Moreover, the approximate equations and solutions for the plate under transverse loadings and magnetic field perturbations are derived directly from the refined plate theory. By omitting higher order terms and coupling effect, the refined plate theory can be degenerated into other well-known elastic and magnetoelastic theoretical models.