With the addition of Na⁺ and Bi³⁺ to the lead-free double perovskite Cs₂AgInCl₆, high photoluminescence quantum yields (PLQYs) can be achieved, making Cs₂AgInCl₆ more promising; however, it is still difficult to achieve blue-light-excited narrowing emission. In this study, Cs_2−xK_xAg_0.6Na_0.4In_0.8Bi_0.2Cl_6−xBr_x was developed with K⁺ and Br⁻, realizing blue-light-excited narrowing red photoluminescence under cryogenic temperatures. The transitional non-localized donor energy level was formed by KBr addition, changing the exciton excitation and transition process. With the increase in the x value, Cs_2−xK_xAg_0.6Na_0.4In_0.8Bi_0.2Cl_6−xBr_x can be excited by blue light at 150 K and emit a narrowing red light. When 1 : 1 KBr was added, the full width at half maximum (FWHM) of Cs_2−xK_xAg_0.6Na_0.4In_0.8Bi_0.2Cl_6−xBr_x red light emission was further narrowed to only 73 nm, when excited by 467 nm blue light at 10 K. This study will stimulate further research on lead-free double perovskites in the field of display technologies.