Metal halide perovskite light-emitting diodes have attracted much attention due to their excellent characteristics such as low-cost solution-processing, high luminous efficiency and excellent color purity. However, low luminous efficiency and spectrum stability of blue perovskite light-emitting device restrict the further development of perovskite materials in the field of displays and lighting. Here in this work, we study the effects of ammonium thiocyanate (NH₄SCN) addition on the morphology, crystal structure, photo-physics, charge transport and electroluminescence properties of quasi-two-dimensional mixed-halide perovskite films by measuring scanning electron microscope (SEM), X-ray diffraction (XRD), UV-Vis spectrum, steady-state photoluminescence (PL), and transient PL and analyzing the current density–voltage characteristics of hole-dominated device and current density-voltage-luminance plots of light-emitting device. The results indicate that ammonium thiocyanate (NH₄SCN) can effectively passivate the defects, improve the crystallinity, and modulate the phase distribution of quasi-two-dimensional mixed-halide perovskite film, thereby increasing charge transport and luminescent efficiency. Notably, PL intensity of the 20%-NH₄SCN sample is 1.7 times higher than that of the control sample, which is attributed to the defect passivation effect of NH₄SCN probably due to the Lewis acid-base interaction with Pb²⁺. Meanwhile, the hole mobility of the 20%-NH₄SCN sample is measured to be 1.31 × 10^–5 cm²/(V·s), which is much higher than that of the control sample (3.58 × 10^–6 cm²/(V·s)). As a result, sky-blue quasi-two-dimensional mixed-halide perovskite light-emitting diode with 20%-NH₄SCN possesses an EL maximum at 486 nm and a maximum external quantum efficiency (EQE) of 5.83% and a luminance of 1258 cd/m², which are 6.7 and 3.6 times higher than those of the control device without NH₄SCN, respectively. At the same time, the EL spectra of the 20%-NH₄SCN device are barely changed under different operating voltages, whereas the EL spectra of the control device show a 7–10 nm red-shift under the same condition, indicating that the NH₄SCN addition inhibits halide phase separation and improves the EL spectrum stability. In addition, the T₅₀ operational life-time of the 20%-NH₄SCN device is measured to be about 110 s, which is superior to that of the control device (39 s) due to improved film quality of NH₄SCN-modified sample. This research provides a simple and effective method to improve the performances of quasi-two-dimensional mixed-halide perovskite blue-emitting diodes.