Slag forming agent for dephosphorization of medium-high-phosphorus semisteel and preparation method of slag forming agent

The invention provides a slag forming agent for dephosphorization of medium-high-phosphorus semisteel and a preparation method of the slag forming agent. The slag forming agent comprises chemical components in percentage by mass as follows: 20-35% of CaO, 5-10% of MgO, at least 10% of FeO, 4-10% of Al2O3 and 10-20% of SiO2. The slag forming agent for the dephosphorization of the medium-high-phosphorus semisteel can be used for accelerating the slag formation in a dephosphorization steelmaking process of the medium-high-phosphorus semisteel in a top-blown converter, and can be used for degreasing the phosphorous in the medium-high-phosphorus semisteel with the phosphorous content of 0.25-0.80 to less than 0.035% by using a Double Slag Process, so the slag forming agent is very important for comprehensively utilizing high-phosphorus sefstromite, solving an ore source shortage situation and reducing the production cost and the steel consumption cost of users.