Perovskite nanocrystals (PNCs) have excellent optical and optoelectronic properties, but their intrinsic instability hampers their practical applications. Herein, stable CsPbBr 3 nanocrystals (NCs) are fabricated with triethylaluminium (TMA, a Lewis acid) and hydrobromic acid by the co-assisted transformation of Cs 4 PbBr 6 NCs. TMA forms a cross-linked alumina (AlO x ) encapsulation layer on the nanocrystal surface to suppress the deformation and ion migration. The introduction of hydrobromic acid acts as a binding ligand, and the acidified reaction environment provides conditions for the water-triggered phase transformation of Cs 4 PbBr 6 NCs into CsPbBr 3 NCs. The synergistic effect of TMA and hydrobromic acid improves the stability of CsPbBr 3 NCs. The obtained CsPbBr 3 NC film maintains a high photoluminescence (PL) intensity after immersion in water. When stored in the atmosphere for over 30 days, the PL intensity of the CsPbBr 3 NC film hardly decreases. The proposed acid co-assisted phase transformation strategy provides a new avenue for the stabilization of PNCs which exhibits wider application prospects in backlight displays.