Abstract: Based on the cluster approach, the high-order perturbation formulas for a 3d9 ion in tetragonal octahedral are established and applied to the studies of the spin Hamiltonian parameters of the g facts g|| and g⊥ and the hyperfine structure constants A|| and A⊥. In these formulas, the contributions to the g factors from the tetragonal distortion are characterized by the tetragonal field parameters DS and Dt. Since the spin Hamiltonian parameters are sensitive to the local structure of a paramagnetic impurity center, the defect structure of Cu2+ center in Ca(OD)2 crystal is estimated, the oxygen octahedron is found to undergo the local elongation ΔZ (0.008 3 nm) along the C4 axis. Based on the above local structure, the calculated results show good agreement with the experimental data, and the defect structure of Cu2+ center in Ca(OD)2 crystal.