The invention relates to a method for controlling phosphorus in the production of ultra-low phosphorous steel by microalloy steel, high carbon steel and special steel, in particular to a process method for producing the ultra-low phosphorous steel by external refining dephosphorization of molten steel, which comprises the following steps of: 1) converter tapping: the temperature is between 1,550 and 1,660 DEG C, the content of the phosphorus is less than 0.02 percent, and the nondeoxidation and the slag-stopping treatment are adopted for tapping; 2) initial dephosphorization: dephosphorizing flux is added into a ladle with steel stream during the converter tapping, and the adding amount is between 5 and 20kg/t; 3) deep dephosphorization: the positions of a blanking tube and a descending tube are adjusted to ensure that the descending tube is positioned at the opposite side of the blanking tube, a ladle is transported to a RH station and is pumped into vacuum, compound spheres are added from a RH high-level bin with a circulation for 0.1 to 15min; 4) slag skimming: the vacuum breaking and the slag skimming are performed , and ladle covering flux is added; 5) heating and stirring: the heating and stirring are performed in a refining furnace, and the heating temperature is controlled to between 1,580 and 1,620 DEG C; 6) the deoxidation and the alloying refining treatment are performed; and 7) continuous casting: the continuous casting adopts the whole-process protection.