The high-order perturbation formulas of electron paramagnetic resonance (EPR) parameters (g factors g ∥, g ⊥ and zero-field splitting D) for 3d 8 ions in trigonal octahedral clusters are established. These formulas contain the contributions not only from the crystal-field (CF) mechanism, but also from the charge-transfer (CT) mechanism (which is not considered in the widely used CF theory). From these formulas, the EPR parameters and the impurity-induced defect structures for Ni 2+ ions in CdX 2 (X=Cl, Br) crystals are studied. The calculated EPR parameters are coincident with the experimental values, and the defect structure of Ni 2+ impurity center obtained from the calculation is different from the corresponding structure in the host crystal. The sign of Q CT (Q=Δg ∥ , Δg ⊥ , or D) due to CT mechanism agrees with that of the corresponding Q CF due to CF mechanism and the relative importance of CT mechanism (characterized by Q CT /Q CF ) increases with increasing covalence of 3d 8 clusters and hence with raising atomic number of ligand X. So, in the explanations of the EPR parameters of 3d 8 (or other 3d n ) ions in crystals with the heavy-element ligand ion (e.g., Br − ), the calculated formulas based on the two-mechanism (CF and CT mechanisms) model are preferable to those based on only the CF mechanism in the CF theory.