The invention relates to a semisteel dephosphorization method, which belongs to the technical field of steel and iron metallurgy and solves the technical problem of high semisteel dephosphorization cost in the prior art. The semisteel dephosphorization method disclosed by the invention comprises the following steps of after the semisteel is mixed into a converter, adding active lime, high-magnesium lime, compound slag formers and converter waste slag; meanwhile, supplying oxygen by an oxygen gun; performing blowing on the dephosphorization materials. The method has the advantages that the steel and iron consumption in the converter steel making process is reduced; the production cost is reduced; the ton steel benefits can reach 8 Yuan/t steel. The method has a good dephosphorization effect; the terminal point molten steel phosphorus content can be lowered to 0.0075 percent; the dephosphorization rate can reach 90.3 percent.