Defect structures and spin Hamiltonian parameters (SHPs) and the corresponding composition dependencies are quantitatively analyzed for divalent copper in ([Formula: see text])Bi2O3–[Formula: see text]Li2O–30(ZnO–B2O3) ([Formula: see text] mol.%) glasses by means of the relevant perturbation formulas of an octahedral 3d9 cluster with tetragonal elongation distortion. The [CuO6][Formula: see text] groups are subject to the moderate tetragonal elongation distortions of 4% due to the Jahn–Teller effect. The cubic crystal-field (CF) parameter, orbital reduction factor, tetragonal elongation ratio and core polarization constant exhibit respective piecewise linear functions of Li2O composition [Formula: see text], which suitably account for the experimental optical spectra and the SHPs at various compositions. Mechanisms of these composition dependencies for the related quantities can be explained as the variations of immediate CFs strength and copper–oxygen electron cloud admixtures with increasing Li2O composition.