Abstract: Based on the crystal field theory, the anisotropic g factors g_ii and the hyperfine structure constants A_ii for (NH₄)₂Mg(SO₄)₂·6H₂O:Cu²⁺ are theoretically investigated by using the high-order perturbation formulas of these electron paramagnetic resonance (EPR) parameters for a 3d⁹ ion in rhombically (C_4v) elongated octahedra. In the calculated formulas, the crystal field parameters are set up from the superposition model and the admixture of d-orbitals in the ground state wave function is taken into account. According to the calculations, the local structure values Cu(H₂O)₆²⁺ clustering in (NH₄)₂Mg(SO₄)₂·6H₂O crystal are: R_x≈0.1872 nm, R_y≈0.2033 nm, R_z≈0.2292 nm; and the mixing coefficients of the ground state wave function parameters α and β are 0.995 and 0.0999, respectively. The calculated results show good agreement with the experimental data, and the reasonableness of these results is discussed.