The invention provides a dephosphorization method for molten iron, which mainly aims at the dephosphorization of the molten iron of a large converter. The method comprises the following steps of: (1) supplying oxygen to the surface of the molten iron from the top through an oxygen lance, wherein the oxygen supply strength is changed in a range of 0.8 to 1.1 Nm<3>/t/min; (2) adding slag forming materials mainly comprising lime, light burned dolomite and ore into the molten iron through a hopper arranged above the converter to perform slag formation, wherein the binary alkalinity (CaO/SiO2) of the formed slag is 1.6 to 2.0; and (3) supplying stirring gas to the interior of the molten iron through a gas supply system at the bottom of the converter, wherein the gas supply strength is 0.25 to 0.40Nm<3>/t/min. The method promotes slag formation and oxygen supply by using low melting point of the low-alkalinity slag, easy slag forming property due to low temperature of the molten iron and large amount of the ore, prolongs the blowing time by using low oxygen supply strength, realizes full mix between slag and metal by using high bottom blowing strength, realizes dephosphorization, and has the advantages of good dephosphorization effect, low lime using amount, low consumption of accessories for dephosphorization and little slag discharge.