High-order perturbation formulas of the electron paramagnetic resonance (EPR) parameters for a 3d 9 ion in a tetragonally elongated octahedron (TEO) are applied to theoretical studies of the EPR parameters (i.e., g factors g || , g ⊥ and the hyperfine parameters A || , A ⊥ ) and local structure for Cu 2+ center in Mg(CH3COO)2• 4H2O (MAT). In the calculated formulas, the ligand orbital and spin-orbit coupling contributions are taken into account due to moderate covalency for the studied systems, and the related molecular orbital coefficients are acquired from the superposition model which enables to correlate the crystal-field parameters and hence the EPR parameters with the local structure of [CuO6] 10− cluster. According to the calculations, the octahedral [CuO6] 10− cluster is found to suffer the relative elongation by about 3.7% along C4 axis in MAT crystal, due to the Jahn-Teller effect. The calculated results are in good agreement with the observed values.