Abstract: According to the calculation with current thermodynanics data, the oxygen content is controlled by the carbon content when the refining process is carried on in a condition of the vacuum degree being about 67 Pa and the carbon content being about 0.2% as well as aluminum content about being large than 0.025%.Because the sulfur content has a fixed relationship with soluble oxygen, the sulfur content is also controlled by vacuum degree and the carbon content. It is proved by the calculation that increasing vacuum degree is benefit for the increasing of stirring power and the decreasing of soluble oxygen content,so the conditions of thermodynamics and dynamics for desulphurization are provided.The results calculated are testified by the results of commercial refining of steel.