Perovskite light-emitting diodes (PeLEDs) have attracted great research interests considering their excellent luminescent properties and solution processability. Despite rapid advances of green-, red-, and near-infrared-emitting PeLEDs, blue-PeLEDs, as an essential part for full-color display and solid-state lighting, still remain challenging due to their low efficiency and spectral instability. Here, reported are spectrally stable blue-PeLEDs biased by an alternating voltage. First, 2-phenoxyethylamine-passivated CsPbBrxCl3− x is obtained as a blue emitter with a record photoluminescence quantum yield of 88%. Subsequently, constructed and optimized are pure blue-emitting PeLEDs exhibiting electroluminescence (EL) at 468 nm with a high external quantum efficiency of 0.71%. Furthermore, driven are the devices by square-wave alternating voltage and stabilized are the EL spectra for 12 h by suppressing the detrimental halide migration during operation. It is believed that this work provides an alternative way for the spectrally stable mixed halide blue PeLEDs.