As a composition-modulation strategy, NH4+-doped (NH4)xCs3-xCu2I5 (0≤x≤3) perovskites using NH4I as the dopant were successfully synthesized via eco-friendly ball milling method in this work. As the ammonium content increases, the lattice constants of (NH4)xCs3-xCu2I5 shrink and the grain sizes increase. The doping of NH4+ effectively passivates the lattice defects, suppresses the non-radiative recombination and tunes the energy band structure, resulting in better fluorescence properties. Among doped and undoped Cs3Cu2I5, (NH4)0.6Cs2.4Cu2I5 and (NH4)0.9Cs2.1Cu2I5 exhibit the highest fluorescence intensity at 450 nm, with an absolute 12.2% improvement in the photoluminescence quantum yield (PLQY) compared with that of pure Cs3Cu2I5. Finally, the UV-pumped deep-blue LEDs based on (NH4)xCs3-xCu2I5 phosphors were fabricated, showing improved performance and tunable emission spectra. These results demonstrate the potential of inorganic NH4+-doping strategy for improving the performance of lead-free perovskite optoelectronics.