In order to reduce the surface defects of ultra-thin stainless steel plate,and choose the appropriate process for304 stainless steel which can meet requirements of desulfurization as well as inclusions plasticization,the activity of component in refining slag for 304 stainless steel and the predicted sulfur distribution ratio LSwere calculated based on the ion and molecule coexistence theory. The measured LSand the calculated LSwere compared,and the calculated LSof AOD desulfurization stage and LF final stage were found to be in good agreement with the measured LS. The chemistries of slag was investigated with variation of the basicity of refining slag. With the basicity increasing from 1.4 to 2.4,the activity of Ca O increased linearly. The lowest theoretical R of the refining slag in different conditions is given,which has certain guiding significance to the actual production. Due to the poor ability of low basicity slag for desulfurization,the operation process of AOD refining with high basic slag and LF refining with low basicity slag was put forward,and the industrial experiment was verified. The result shows that the mass percent of sulfur meets the requirements and inclusions in steel show low liquidus temperature. This process provides the possibility of reducing the surface defects for ultra-thin stainless steel plate.