The invention discloses an ultra-low phosphorus steel smelting method. Dephosphorizing pretreatment is conducted firstly, and then operation is conducted through a double slag method. A dephosphorization period and a blowing period are included. According to the dephosphorization period, a dephosphorization agent and pre-slag left by a previous converter serve as slagging materials to be subjectedto dephosphorization smelting, and alkali converter slag is obtained; according to the blowing period, primary blowing is conducted, specifically, the oxygen flow rate during primary blowing is appropriately decreased, the converter bottom blowing strength is larger than or equal to 0.045 Nm<3> / (t.min), the oxygen-supplying blowing time is 6-7 min, after primary blowing is completed, converter slag is discharged, and then new slag is added to be slagged again; and the final converter slag alkalinity of secondary blowing is controlled to be 3.5-4.5, the content of FeO is 18-25%, the content ofMgO is 8-10%, the final temperature is 1600-1630 DEG C, the content of final C is 0.03-0.06%, a special slag-stopping plug is adopted before steel taping, after 3 / 4 steel taping is conducted, a slag-stopping awl is additionally arranged, and 40-60% of later-period slag which is generated after secondary blowing is finished is left to serve as a slagging material of later converter steel. Throughthe technology, the converter final P can be stably controlled to be 0.005% or below.