CsPbBr₃-Cs₄PbBr₆ dual-phase nanocrystals are prepared by adding the mixture ligand of oleylamine and tetradecyl-phosphonic acid (OLA-TDPA) to CsPbBr₃ perovskite nanocrystals through ligand post-treatment. The structure, the morphology, optical property and the stability of CsPbBr₃-Cs₄PbBr₆ dual-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 CsPbBr₃, 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 CsPbBr₃-Cs₄PbBr₆ nanocrystals. In the first stage (t ≤ 1 h), the m OLA-TDPA mixing ligand can form (RNH₃)₂PO₃ X type ligand and exchanges with RNH₃⁺-RCOO^– at the surface of CsPbBr₃ nanocrystals, which can effectively passivate surface defects by strong interaction with Pb²⁺ and high ligand content at surface, thus improving the quantum yield and fluorescence life of CsPbBr₃ nanocrystals with spherical shape. In the second stage, with the increase of reaction time, PbBr₂ partially dissolves from the surface of CsPbBr₃ nanocrystals, then some CsPbBr₃ nanocrystals transform into lead-depleted Cs₄PbBr₆ nanocrystals 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.