All-inorganic CsPbI perovskite quantum dots (PeQDs) have sparked widespread research due to their excellent optoelectronic properties and facile synthesis. However, attaining highly stable CsPbI perovskite quantum dots (PeQDs) against heat and polar solvents still remains a challenge and hinders any further practical application. Here, by exploiting (3-aminopropyl) triethoxysilane (APTES) as the sole silica (SiO) precursor, we report a one-step in situ synthesis of single SiO-coated CsPbI (SiO-CsPbI) PeQDs, namely that one SiO particle only contains one CsPbI PeQD particle. The obtained SiO-CsPbI PeQDs are cubic in shape, have a more uniform size distribution, and possess narrow emission, with near unit photoluminescence quantum yields of up to 97.5%. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirm the formation of SiO through the hydrolysis of APTES on the CsPbI PeQDs surface. Furthermore, they have a significantly improved stability against storage, heat, and ethanol. By combining purple-emission GaN light-emitting diodes, the SiO-CsPbI PeQDs were successfully employed as down-conversion emitters and exhibited considerable enhanced luminous performance and excellent stability, demonstrating their promising future in the practical application of solid-state lighting fields.