The invention belongs to the field of metallurgy, and relates to a method for producing low phosphorus steel by smelting semisteel, which solves the technical problem of controlling the steelmaking end point of a converter within 0.006 percent. The method is mainly realized by adjusting slag-making parameters of the converter smelting by adopting a single slag method, which comprises that: the slag-making materials added in the converter slag-making process consist of 40-50kg of active lime, 20-30kg of high-magnesium lime and 15-25kg of composite slag former based on per ton of tapping moltensteel; and the slag-making materials are added in twice. In addition, by controlling an oxygen supply system and an end point control system, the dephosphorization rate of the converter can reach over 92 percent; molten steel containing less than 0.006 percent of phosphorus is obtained; steel ladle slag de-P and alloy re-P are controlled within 0.002 percent. The method can stably produce finished low phosphorus steel containing less than 0.010 percent of phosphorus, and is characterized by simple operation method, small investment on equipment and low production cost.