Abstract: Xinggang one-step method (dephosphorization station +60 t AOD furnace +60 t LF furnace) is used in the production of 400 series free cutting stainless steel, in the early stage, in all the sulphur iron was added to the steel in the AOD to fit [S]. The alkalinity of steel slag during the AOD tapping to the upper casting process is always in the low alkalinity range (R = 1.40 ~ 1.95). The consumption of sulphur iron is large, oxygen content in molten steel is high, with the increase of the number of smelting furnaces, the lining erosion is serious, effecting AOD furnace life and billet quality and the steel slag is in a low alkalinity state for a long time, which is easy to cause the increase of [C] content in the steel (especially 430F, 430FR low carbon steel) , which is difficult to achieve multi-furnace continuous pouring. In the later stage, by optimizing the method of adding iron sulphide and the alkalinity of slag at different stages, including iron sulphide in LF late period, AOD slag basicity 2.0 ~ 2.3 and LF slag basicity 1.6 ~ 2.0, the treatment time of low alkalinity slag is shortened, the [S] element loss and the oxygen content of molten steel and the erosion of the furnace lining are reduced. The sulfur element absorption rate was increased from 62% to 75% , the sulphur iron consumption per ton of steel decreased by 2.12 kg, and defects such as subcutaneous bubbles in the slab were controlled.