Colloidal all‐inorganic perovskites nanocrystals (NCs) have emerged as a promising material for display and lighting due to their excellent optical properties. However, blue emissive NCs usually suffer from low photoluminescence quantum yields (PLQYs) and poor stability, rendering them the bottleneck for full‐color all‐perovskite optoelectronic applications. Herein, a facile approach is reported to enhance the emission efficiency and stability of blue emissive perovskite nano‐structures via surface passivation with potassium bromide. By adding potassium oleate and excess PbBr2 to the perovskite precursor solutions, potassium bromide‐passivated (KBr‐passivated) blue‐emitting (≈450 nm) CsPbBr3 nanoplatelets (NPLs) is successfully synthesized with a respectably high PLQY of 87%. In sharp contrast to most reported perovskite NPLs, no shifting in emission wavelength is observed in these passivated NPLs even after prolonged exposures to intense irradiations and elevated temperature, clearly revealing their excellent photo‐ and thermal‐stabilities. The enhancements are attributed to the formation of K‐Br bonding on the surface which suppresses ion migration and formation of Br‐vacancies, thus improving both the PL emission and stability of CsPbBr3 NPLs. Furthermore, all‐perovskite white light‐emitting diodes (WLEDs) are successfully constructed, suggesting that the proposed KBr‐passivated strategy can promote the development of the perovskite family for a wider range of optoelectronic applications. High‐quality blue‐emitting CsPbBr3 nanoplatelets (NPLs) are synthesized via a facile potassium bromide‐enriched surface passivation. The resultant blue‐emitting (≈450 nm) CsPbBr3 NPLs show a high PLQY of 87% with excellent thermal stability and photostability. Furthermore, white light LEDs based on the mixture perovskite materials including the blue‐emitting NPLs are constructed, demonstrating a wide color gamut.