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In Tb-Dy-Fe alloy systems, Tb_0.29Dy_0.71Fe_1.95 alloy shows giant magnetostrictive properties under low magnetic fields, thus having great potential for transducers, microsensors, and other applications. The C15 cubic crystal structure of Tb-Dy-Fe has long been thought to be the source of giant magnetostriction. It is surprising that such a highly symmetrical crystal structure exhibits such a large magnetostrictive strain. In this work, the lattice parameters of Tb_0.29Dy_0.71Fe_1.95 magnetostrictive materials were studied by processing atomic-resolution images. The selected area diffraction patterns show a face-centered cubic structure, but the fast Fourier transform diagram shows that the cubic structure has obvious distortion. The lattice parameters obtained by geometric phase analysis (GPA) and Gaussian model-based fitting and calculation show that the lattice constants a, b, and c are not strictly equal, and small disturbance of the lattice constants occurs based on the cubic structure. The actual crystal structure of the Tb-Dy-Fe material is a slightly disturbed cubic structure. This variation in the crystal lattice is mainly caused by the inhomogeneous composition and may be related to the giant magnetostrictive properties of Tb-Dy-Fe alloy.