For the first time, a novel class of deep-blue (DB)-emitting Cs_x(NH₄)_1−xPbCl₂Br (0.3 ≤ x ≤ 1) perovskite nanocrystals (PNCs) were prepared by a facile ligand-assisted one-step ball milling method. The resulted PNCs are characterized by high chlorine content (66.7%) and excellent color purity. Their photoluminescence position can be finely modulated from 434 nm to 447 nm, which extends notably beyond the current Rec. 2020 color standard, by the NH₄⁺ content. Among them, Cs_0.3(NH₄)_0.7PbCl₂Br shows the highest quantum yield close to 40%. The PNCs exhibit high phase and optical stability under ambient conditions and UV light according to the NH₄⁺ content. This work offers a new avenue to produce DB perovskites for future full-color displays and optoelectronics.