The invention provides a converter smelting method for ultra-low-phosphorus steel. According to furnace feeding molten iron conditions, the content of C ranges from 4.0% to 4.3%, the content of Si ranges from 0.20% to 0.50%, the content of P is smaller than or equal to 0.065%, and the content of S is smaller than or equal to 0.002%; the temperature of the molten iron ranges from 1,260 DEG C to 1,280 DEG C; and fluorspar, dolomite and metallurgy lime are added before slagging, alkalinity ranges from 2.5 to 2.8, the gun position is adjusted according to the blowing time, and the oxygen supply intensity, the catch carbon time, oxygen accumulation, the molten steel temperature, the molten iron carbon content and the molten iron phosphorus content are controlled. After primary point blowing and secondary point blowing, the metallurgy lime is added, and the alkalinity, the gun position, the oxygen supply intensity, the catch carbon time, the molten steel temperature, the molten iron carbon content and the molten iron phosphorus content are controlled. Slag blocking tapping is adopted, and molten steel drossing is conducted after tapping. Through optimization of converter smelting technology parameters, the phenomenon of peroxidation tapping caused by smelting of a duplexing process can be effectively avoided, a double-slag process is adopted for smelting ultra-low-phosphorous steel, and the content of the tapping phosphorus is stably controlled to be below 0.0035%.