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The optical absorption and emission properties of 20Li2O-xBi2O3-(78-x)TeO2-1Er2O3-1Ag glass system had been analyzed to investigate the hypersensitivity shift and its mechanism. According to the Covalent model, the hypersensitive shift indicated by the drop of F2 at x = 5 mol% can be attributed to charge transfer from O2– ligands to Er3+ ions meanwhile according to the Dielectric screening model, drop of F2 may due to Er3+ ions contraction. The dynamic coupling mechanism was used to explain hypersensitivity transition probabilities in non-centrosymmetric systems. The addition of Bi2O3 may modify the site symmetry of Er3+ ions and oxygen to a high asymmetry τ2, resulting in an asymmetrical electron distribution, thus increasing Er-O covalency, as shown by the maximum Ω2 value at x = 5 mol%. The Er3+ ions site symmetry was investigated using Hamiltonian crystal field fitting in the frame of the D4 point symmetry model, which yielded maximum crystal field strength Nv at x = 5 mol%, indicating low point symmetry distortion of the Er3+ ions site symmetry.