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Phosphors with a longer excitation wavelength exhibit higher energy conversion efficiency. Herein, quantum cutting KGd(CO₃)₂:Tb³⁺ phosphors excited by middle-wave ultraviolet were synthesized via a hydrothermal method. All the KGd(CO₃)₂:<i>x</i>Tb³⁺ phosphors remain in monoclinic structures in a large Tb³⁺ doping range. In the KGd(CO₃)₂ host, ⁶D_3/2 and ⁶I_17/2 of Gd³⁺ were employed for quantum cutting in sensitizing levels. The excited state electrons could easily transfer from Gd³⁺ to Tb³⁺ with high efficiency. There are three efficient excited bands for quantum cutting. The excited wavelengths of 244, 273, and 283 nm correspond to the transition processes of ⁸S_7/2→⁶D_3/2 (Gd³⁺), ⁸S_7/2→⁶I_17/2 (Gd³⁺), and ⁷F₆→⁵F₄ (Tb³⁺), and the maximum quantum yields of KGd(CO₃)₂:Tb³⁺ can reach 163.5, 119, and 143%, respectively. The continuous and efficient excitation band of 273–283 nm can well match the commercial 275 nm LED chip to expand the usage of solid-state light sources. Meanwhile, the phosphor also shows good excitation efficiency at 365 nm in a high Tb³⁺ doping concentration. Therefore, KGd(CO₃)₂:Tb³⁺ is an efficient green-emitting phosphor for ultraviolet-excited solid-state light sources.