Based on thermodynamics, a constitutive model for magnetic shape memory alloy (MSMA) is developed under biaxial magnetic field. Considering the driving force provided by Gibbs free energy and resistive force during martensite reorientation, a kinetic equation is got. Of special concern is the influence of biaxial magnetic field on martensite reorientation for a Ni2MnGa single-crystal specimen. The theoretical results are in agreement with experimental data. Some useful conclusions can be obtained.