The vacancy-ordered quadruple perovskite Cs₄CdBi₂Cl₁₂, as a newly-emerging lead-free perovskite system, has attracted great research interest due to its excellent stability and direct band gap. However, the poor luminescence performance limits its application in light-emitting diodes (LEDs) and other fields. Herein, for the first time, an Ag⁺ ion doping strategy was proposed to greatly improve the emission performance of Cs₄CdBi₂Cl₁₂ synthesized by hydrothermal method. Density functional theory calculations combined with experimental results evidence that the weak orange emission from Cs₄CdBi₂Cl₁₂ is attributed to the phonon scattering and energy level crossing due to the large lattice distortion under excited states. Fortunately, Ag⁺ ion doping breaks the intrinsic crystal field environment of Cs₄CdBi₂Cl₁₂, suppresses the crossover between ground and excited states, and reduces the energy loss in the form of nonradiative recombination. At a critical doping amount of 0.8%, the emission intensity of Cs₄CdBi₂Cl₁₂:Ag⁺ reaches the maximum, about eight times that of the pristine sample. Moreover, the doped Cs₄CdBi₂Cl₁₂ still maintains excellent stability against heat, ultraviolet irradiation, and environmental oxygen/moisture. The above advantages make it possible for this material to be used as solid-state phosphors for white LEDs applications, and the Commission International de I’Eclairage color coordinates of (0.31, 0.34) and high color rendering index of 90.6 were achieved. More importantly, the white LED demonstrates remarkable operation stability in air ambient, showing almost no emission decay after a long working time for 48 h. We believe that this study puts forward an effective ion-doping strategy for emission enhancement of vacancy-ordered quadruple perovskite Cs₄CdBi₂Cl₁₂, highlighting its great potential as efficient emitter compatible for practical applications.