Abstract: The surface defects of ML08Al billet produced by a Chinese steel plant were dominated by angle hook type of oscillation mark and surface concave. Through the analysis of composition and physicochemical properties of the used mould flux,it was found out that the high level of Fe₂O₃ in mould flux would reduce the heat retention,and the liquid slag layer would be thinner. The higher melting point and the fractional melting phenomenon of the mould flux influenced the melting homogeneity of the slag,and the slag-ring readily formed. The mould flux was not easy to form crystallization,this would deteriorate the uniform heat transform for ML08AL initial solidification in the mould. In this research,based on the quality problem of the casting billet,the requirements for physicochemical properties of the mould flux had been fixed. Combining with the calculation of the molten steel solidification in mold via thermal coupling model,the mould flux had been modified in terms of melting point,viscosity,melting rate and alkalinity of the mould flux. The surface quality of the casting billet had been improved significantly by using the optimized mould flux.