Ferromagnetic shape memory alloys (FSMAs), such as Ni₂MnGa alloy, promise to be the keymaterials in manufacturing new type of actuator and sensor because of their high responsive speed and large strainoutput. In order to find the best functions of FSMAs, the understanding of their martensite transformationcrystallography is of enormous necessity and importance. In the present study, the topological model of martensite transformation was applied to the analysis of the interphase interfacial defect structure and crystallography ofmartensite transformation, such as the habit plane index and orientation relationship, in a Ni₂MnGa alloy. The results obtained in this work were compared with the prediction from the phenomenological theory of martensite crystallography (PTMC). When the transformation dislocation b_+1/+1^D1 with smaller Burgers vectors and the lattice invariant deformation (LID) dislocation b^L=0.186[111]_M are activated to accommodate the coherency strain with a twist angle ω=3.2°, the habit plane is determined to be {0.691-0.117 0.713}_p represented in the parent crystal frame, having an inclination angle Ψ_p=42.000°with the terrace plane (111)_p. Furthermore, the orientation relationship between the parent and martensite phases is found to be, namely, {0.69-0.117 0.713}_p 0.317° away from (101)_M and [110]_p about 3.200° away from [111]_M, which are close to the calculated values based on the phenomenological theory of martensite crystallography. It is evident that the transformation dislocation with smaller Burgers vectors can be activated more easily during a martensite transformation. Additionally, for a range of twist ω, the alternative combination of disconnection and LID might be introduced in the topological model to obtain multiple predictions of martensite transformation crystallography, which could provide an explanation for the diversity and complexity of martensite transformation crystallography resulting from the different measured positions or external fields. Moreover, the parameters of the interfacial defect network, i.e., the line direction and the spacing of the interfacial defects, were also determined according to the topological model of martensite transformation, while the PTMC provides limited information of the microstructure of the martensite interface. It has been shown that the topological model of martensite transformation exhibits greater flexibility and advantage compared to the PTMC.