The invention discloses a semi-steel steelmaking method, wherein the method comprises the steps of: adding raw materials into a steel making furnace, blowing oxygen into the furnace to obtain molten steel and slag; the raw materials comprises raw steel, lime, high-magnesium lime and acid composite slag; the raw steel contains semi-steel after vanadium extraction; the addition of the acid compositeslag meets the following formula: W (acid composite slag) is equal to (( (CaO (lime) plus CaO (high-magnesium lime) minus SiO2 (oxidized semi-steel) multiplied by R) divided by R ) divided by SiO2% (acid composite slag) ) multiplied by K, wherein W acid composite slag refers to the addition of the acid composite slag, CaO lime refers to the quantity of CaO brought in by the lime, CaO high-magnesium lime refers to quantity of CaO brought in by the high-magnesium lime, SiO2 oxidized semi-steel refers to the quantity of SiO2 converted from the quantity of Si element in the semi-steel, R refers to the alkalinity of slag, and ranges from 3 to 5, SiO2% acid composite slag refers to the weight percentage of SiO2 in the acid composite slag, and K refers to a fluctuation coefficient with a value ranging from 0.98 to 1.02. The semi-steel steelmaking method can achieve good dephophorization and desulfurization and is applicable to the automation production.