<p>For the reason of high Mn content in steel for ultra-low-temperature applications, it is prone to react with conventional mold flux. Consequently, the non-reactive CaO-Al₂O₃-based mold flux is recommended for use in continuous casting processes. However, this slag system has problems such as low consumption and pronounced crystallization ability. To improve viscosity and crystallization characteristics of CaO-Al₂O₃-based mold flux, addition of an appropriate amount of MnO is beneficial. In this study, effect of MnO on viscosity, network structure, and crystallization characteristics of CaO-Al₂O₃-MnO-based mold flux was investigated by using Rotating Cylinder Method, Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), Differential Thermal Analysis (DTA), and X-ray Diffraction (XRD). The experimental results show that when MnO content is incrementally increased to 9 wt pct, both viscosity and activation energy for viscous flow of the slag decrease and subsequently rise, and the minimum value is observed at 6 wt pct. The incorporation of MnO facilitates the formation of aluminate structures and the transformation from [AlO₄]⁵⁻ tetrahedral units to [AlO₆]⁹⁻ octahedral units, resulting in the polymerization degree of the slag reaching its minimum at 6 wt pct MnO. As the addition of MnO, the Ca₁₉(Mn, Al)₁₃Si₁₈O₇₈ phase precipitate in the low-temperature range, and the crystallization time, and crystallization temperature of the slag initially reduces and then increases, with the lowest values occurring at 6 wt pct. Furthermore, the melting temperature of the slag decreases with incorporation of MnO, and the effect is more pronounced when the MnO content is less than 6 wt pct. Consequently, it is recommended that when CaO/Al₂O₃ ratio is fixed at 1.0, the MnO content in CaO-Al₂O₃-MnO-based mold flux of high-Mn steel for ultra-low-temperature applications is maintained at approximately 6 wt pct.</p>