Abstract: Based on analysis of the inclusion restriction in ML08Al steel, the component range of final refining slag was designed and the slag forming system was determined. The productive practice shows that the slag melting condition of refining slag is better and the desulphurizing ratio is relatively high, which can satisfy the requirements of steel grade. The research on cleanliness of billet continuous casting shows that, under the current condition, the TO content is less than 20×10^-6, the inclusion amount of electrolytic is less than 10 mg/10 kg steel. The thermodynamics of Ca treatment was also discussed.