Based on the defect model that the rhombic Dy 3+ center in AgCl crystal is formed by substitutional Dy 3+ ion associated with two nearest Ag + vacancies ( V Ag) along the 〈1 1 0〉 and 〈 1 ¯ 1 ¯ 0 〉 axes owing to charge compensation, the spin-Hamiltonian parameters ( g factors g i and hyperfine structure constants 161 A i and 163 A i , where i = x, y, z) of this rhombic Dy 3+ center are calculated from a diagonalization (of energy matrix) method. In the method, the Zeeman (or magnetic) and hyperfine interaction terms are attached to the classical Hamiltonian used in the calculation of crystal-field energy levels and a 66 × 66 energy matrix concerning this Hamiltonian is constructed by taking all the ground-term multiplets 6 H J ( J = 15/2, 13/2, 11/2, 9/2, 7/2, 5/2) into account. The calculated results ( g factors g i and average | A ¯ ( 161 D y 3 + ) | and | A ¯ ( 163 D y 3 + ) | ) are in reasonable agreement with the experimental values. From the calculations, the above defect model of rhombic Dy 3+ center is confirmed, the defect structure of this Dy 3+ center (characterized by the displacement of Cl − ligand caused by V Ag) is obtained and the components of hyperfine structure constants A i ( 161Dy 3+) and A i ( 163Dy 3+) are predicted. The results are discussed.