Phosphors with high color purity and brightness are strongly desired for applications in areas such as solid-state lighting and displays. Herein, a new narrow-band intense green-emitting phosphor, Eu²⁺ doped CsKNaLi(Li₃SiO₄)₄ (CKNLLSO) with a full width at half maximum of 55 nm, is reported. The substitutions of Eu²⁺ on three monovalent cations Na⁺, K⁺, and Cs⁺ resulted in three Gaussian emission bands. The barycenter energy of Eu²⁺ 4f⁶5d configuration was calculated using a semi-empirical formula and compared with the experimental value. The rigid tunnel structure and luminescence concentration quenching were also analyzed to judge site occupation. The crystal and band structures of the host compound were characterized and calculated with the aid of Rietveld refinement and density functional theory, respectively. The CKNLLSO:Eu²⁺ phosphor exhibits relatively high thermal stability even if the temperature is increased to 190 °C. The optical properties of green and white LEDs fabricated by mixing the optimized CKNLLSO:4%Eu²⁺ phosphor without or with commercial blue and red phosphors were investigated. The detailed discussion about the relationship between the rigid tunnel structure and PL properties of CKNLLSO:Eu²⁺ will serve as a guide to develop new oxide-based narrow-band phosphors. All the results imply the great potential of Eu²⁺ doped CKNLLSO phosphors for applications in the field of backlighting or pc-wLEDs.