Perovskite nanomaterials have been fascinating for commercial applications and fundamental research owing to their excellent optical properties and satisfactory processability. They are expected to be alternative downconversion materials in phosphor-converted LEDs for lighting or display technology. However, owing to their low formation energy and large specific surface area, perovskite nanomaterials are sensitive to environmental stress like humidity, heat, etc. In this paper, cubic CsPbI3 quantum dots (QDs) with improved stability are synthesized using (3-aminopropyl)triethoxysilane (APTES). These luminescent CsPbI, QDs passivated by APTES not only show excellent stability when stored in hexane but also possess outstanding steadiness for lattice structure when prepared as a thin film in open air. They do not decompose immediately in the water. Such excellent stability is attributed to the hindrance from hydrolysis of APTES, which forms an analogous core-shell structure to protect the "core" CsPbI3 QDs. Furthermore, an additional iodine source is added to enhance their emissionm and CsPbI3 QDs with a PLQY of 84% are synthesized.