The invention provides a molten steel dephosphorization agent and a molten steel dephosphorization refining method. The molten steel dephosphorization method comprises the following steps in sequence: ensuring the oxygen activity is greater than or equal to 5000ppm at the final point of molten iron at the blowing end point of a converter, and adding 5-8kg of a dephosphorization agent into each ton of steel in the process that steel is discharged from the converter, wherein the dephosphorization agent comprises the following components in parts by weight: 85-90 parts of lime and 10-15 parts of fluorite, and the total parts is 100; performing LF ladle refining on the molten iron, melting the slag in an electric heating mode, introducing argon for bottom blowing and stirring to the molten iron at the speed of 80-150Nm<3>/hour so as to provide a sufficient power reaction space to the molten iron and the dephosphorization slag, and subsequently slagging off. The molten steel dephosphorization agent has the beneficial effects that dephosphorization treatment can be performed on the molten iron after smelting and dephosphorization in the converter, so that the rephosphorization phenomenon of the molten iron which is smelted in the converter can be effectively avoided, and the dephosphorization effect is good.