Improving the performance of blue light-emitting devices is an urgent requirement for the practical application of perovskite light-emitting devices. In this study, we investigated the effects of the hypophosphorous acid (HPA) additive on the morphology, crystallinity, and photophysical characteristics of quasi-two-dimensional perovskites containing mixed-halide anions and binary long-chain ammonium cations. The results show that the HPA additive can improve the crystallinity of perovskite films, boost the formation of the light emission phase, effectively passivate the defects, and enhance charge transport. The maximum external quantum efficiency and maximum luminance of the quasi-two-dimensional sky-blue perovskite light-emitting devices are 7.9% and 7300 cd/m ², respectively, which are about 4.2 and 2.9 times that of the devices without HPA. The operational stability of the devices is also improved.