Since the perovskite has an easy solution reaction, a reaction at a low temperature, and a suitable band gap for a solar cell, much research has been conducted on a low-cost, high-efficiency solar cell. Recently, a light emitting material has been reported using all-inorganic perovskite nanocrystals. materials due to the characteristics of narrow half-width maximum, and high photoluminescence quantum efficiency, bandgap control in the entire visible light through the halide composition. However, reduction in efficiency due to heat and moisture is preventing commercialization. Recently, a method of improving the structural stability and the fluorescence efficiency by adding a metal salt has been reported. However, the role of metal salts in stabilization is still unknown. The stabilization of cesium lead halide perovskite nanocrystal surfaces were studied by the first-principle calculation method. Based on the Octet rule, the model of the surface stabilized by the ligand was presented and the thermodynamic stability was compared. It was confirmed that the stable surface varies depending on the chemical environment, and the merging of nanocrystals can be explained. In addition, we can understand the interaction between nanocrystals and ligands by presenting a model of colloidal nanocrystals in comparison with experimental data.