Recently, blue perovskite light emitting diodes (PeLEDs) is becoming a research hot spot. However, the electroluminescence (EL) spectra of blue PeLEDs based on Cl/Br mixed perovskites will be redshifted under bias, due to the ion migration of Cl and Br anions, which badly hinders the commercialization of blue PeLEDs. In this work, the cesium trifluoroacetate (CsTFA) is introduced to the CsPbClBr2 perovskite precursor to enhance the nucleation density, leading to the formation of small nanoparticle-based perovskite films and thus enhancing the film morphology. Most importantly, the C=O bonds in CsTFA can function effectively in passivating the defects at grain boundaries and preventing the ion migration of halide anions, which largely stabilize the EL spectra. The EL spectra can maintain even under high driven voltage. The maximum luminance and the external quantum efficiency (EQE) is enhanced by 31- and 8-fold compared with control devices, respectively.