The invention provides a composite slagging agent for semi-steel making. The composite slagging agent contains iron oxide, MnO and SiO2, wherein the iron oxide is Fe2O3 and/or FeO. The invention also provides a preparation method for the composite slagging agent and a semi-steel making method. According to the composite slagging agent provided by the invention for semi-steel making, the preparation method is simple, the consumption of the composite slagging agent in the semi-steel making process is low, and the composite slagging agent can be used for slagging and dephosphorization in the semi-steel making so that the semi-steel making process is performed more smoothly; and meanwhile, the composite slagging agent can effectively improve the Mn element content of the final molten steel and correspondingly reduce the consumption of Mn-containing alloy added in a subsequent step of the steel making production so as to reduce the steel making production cost.