In this paper, a novel micromechanical modeling framework is presented to investigate mechanical properties of a multiphase magnetostrictive composite subjected to a multi-field coupling environment. To this end, a nonlinear constitutive equation with consideration of mechanical-magneto-thermal condition is proposed. Parametric elements are used to discretize a representative volume element (RVE) of the material to obtain local stress distribution. The macroscopic strain responses of the magnetostrictive material under magnetic field loading are predicated considering local equilibrium and using the homogenization technique. Numerical results are compared with the available experimental data. In general, the proposed method offers a useful tool to study the effects of external pre-stress, ambient temperature and fiber volume fraction on the overall characteristics of fiber reinforced magnetostrictive composites. The numerical results show that the nonlinear variations of strain and flux density are closely related to the magnetization intensity.