Abstract: According to the steel grade produced at PZH Steel, the effect of main constitution in converter slag on slaging, desulphurizing and dephosphorizing in steelmaking process and on slag's viscosity were studied. The controlled range of (CaO/SiO₂), TFe and MgO in slag was put forward. Based on the trial in industry, the system of slag was set up fitting to steelmaking with semi-steel and slag splashing at the same time.