With nontoxicity and high emission efficiency, luminescent copper(I)-based halides have attracted much attention as alternatives for lead-based perovskites in photoelectric domains. However, extending the emission wavelength by doping with Mn 2+ in a facile way is still a challenge. In this work, Mn 2+ -doped Cs 3 Cu 2 I 5 microcrystals were synthesized by a mild solution method, and double emission bands from self-trapped excitons (STEs) and Mn 2+ peaking at 445 and 560 nm, respectively, were observed. More importantly, further introduction of alkali metal ions (Rb + , K + , Na + ) considerably promoted the luminescence performance of the Cs 3 Cu 2 I 5 -Mn microcrystals. The STE → Mn 2+ energy transfer efficiency of the typical sample doped with Na + increased from ∼0 to 21.30%, and the photoluminescence quantum yield (PLQY) increased from 47.32% to 62.06%. The detailed structural and optical characterizations combined with DFT calculations proved that the doping with alkali metal ions causes lattice distortion and enhances the coupling between [MnI 4 ] and [CuI 4 ] tetrahedra, thus promoting the energy transfer efficiency and the PLQY.