In the Argon Oxygen (Argon Oxygen Decarburization, AOD method) refining medium and low carbon ferrochrome process, the slag basicity is one of the important factors affecting the product quality and energy consumption. Appropriate slag basicity can quickly remove impurities such as sulfur and phosphorus in the liquid steel, and also can reduce the furnace lining erosion degree, reducing the consumption of refractory material at the same time. However, it is difficult to obtain accurate continuous detection information of slag basicity because the smelting ferroalloys, is a very complex nonlinear multivariate polyphase under high temperature condition of physical and chemical reaction process and there are many uncertain factors. The relationship model between the slag basicity and the dephosphorization efficiency was established in the paper based on thermodynamics analysis on dephosphorization process of the AOD low carbon process, using support vector machine (SVM), whose relative error was within 5%, which have a very important significance of improving the dephosphorization efficiency of AOD and solving the problem of high phosphorus content.