[en] In tetragonal ligand-field, based on the Zhao's semi-SCF d-orbit wave functions model, the Macfarlane's perturbation loop method and point-charge crystal field model, the third-order perturbation formulas of electron paramagnetic resonance parameters (zero-field splitting D and g-factors) for 3d^3/7 configuration ions have been constructed. In these formulas, the contributions from configuration interaction, covalency effect, Racah-Trees corrections and local structure distortion (ΔR) have been considered. By using these formulas, the electron paramagnetic resonance parameters of Cr³⁺ ion at tetragonal site in K₂ZnF₄ crystal have been calculated. The theoretically calculated values are consistent with the experimentally observed.