CsPbBr ₃-Cs ₄PbBr ₆dual-phase nanocrystals are prepared by adding the mixture ligand of oleylamine and tetradecyl-phosphonic acid (OLA-TDPA) to CsPbBr ₃perovskite nanocrystals through ligand post-treatment. The structure, the morphology, optical property and the stability of CsPbBr ₃-Cs ₄PbBr ₆dual-phase nanocrystals are characterized by X-ray diffraction, transmission electron microscopy (high-resolution TEM), UV-vis spectrophotometer, fluorescence spectrophotometer, and transient fluorescence spectrophotometer. The as-obtained nanocrystals have a high photoluminescence quantum yield of 78% and long fluorescence lifetime of 476 ns when prepared at the optimal molar ratio of CsPbBr ₃, TDPA and OLA (1∶1∶15). Moreover, the nanocrystal is quite stable at room temperature for at least 25 days, and has a good thermal stability in five heating-cooling cycles at temperature in a range between 293 K and 328 K. The formation of dual-phase nanocrystals go through two stages of surface passivation/dissolution and recrystallization to generate CsPbBr ₃-Cs ₄PbBr ₆nanocrystals. In the first stage ( t≤ 1 h), the m OLA-TDPA mixing ligand can form (RNH ₃) ₂PO ₃X type ligand and exchanges with [RNH ₃] ⁺-[RCOO] ^–at the surface of CsPbBr ₃nanocrystals, which can effectively passivate surface defects by strong interaction with Pb ²⁺and high ligand content at surface, thus improving the quantum yield and fluorescence life of CsPbBr ₃nanocrystals with spherical shape. In the second stage, with the increase of reaction time, PbBr ₂partially dissolves from the surface of CsPbBr ₃nanocrystals, then some CsPbBr ₃nanocrystals transform into lead-depleted Cs ₄PbBr ₆nanocrystals with hexagonal phase, thus improving the stability of nanocrystals. This work has a certain reference value for promoting the applications of high efficient and stable perovskite nanocrystals.