The characteristics of magnetic domain deflection and permeability under piezomagnetic and magnetoelastic effects in Tb0.3Dy0.7Fe2 alloy are studied in this paper. Based on the minimal value principle of Stoner-Wolhfarth model, the curve of free energy versus domain deflection angle is analyzed, and the angle deflections of magnetic domain under differ compressive stresses and magnetic fields are discussed. The calculational and experimental results of permeability under stress and magnetic field loads are used to confirm the analysis of domain deflection in ally. These results indicate that the magnetic domains [111] and [111] each have a transition effect of angle deflection with the increase of stress and magnetic field, which can be used to explain the huge magneticostrictive mechanism of Terfenol-D. Also, permeability has a negative relation with stress and magnetic field, and the influence of magnetic energy on permeability is greater than the stress energy effect, especially under small loads. The experimental results of permeability are in a good agreement with the calculations confirming the validity of calculation and analyzsis. The above computations have a significant guidance for analyzing and studying the magnetic domain deflection model and hysteresis in Terfenol-D.