The invention discloses a semi-steel steelmaking slag former, which belongs to the technical field of metallurgical engineering, and comprises the following raw materials by weight: 75-85 parts of lime, 15-20 parts of magnesium oxide, 0.8-1 part of a binder, 2-3 parts of hydrocarbon particles, and 6-8 parts of an auxiliary agent. And the hydrocarbon particles, magnesium oxide, lime, an auxiliary agent and a binding agent are evenly mixed and then ground and subjected to compression molding, and the slag former is obtained. The hydrocarbon particles and the auxiliaries are added and have a synergistic effect, generation of FeO is promoted, dissolution of CaO is promoted, slagging and dephosphorization reactions can be accelerated, the converter blowing time is effectively shortened, and the overall converter steelmaking efficiency is improved; in addition, no other impurities are introduced into the molten steel, and the quality of steel finished products is improved.