In consideration of requirements for phosphorus content in steel, low slag operation achieved by optimization of steelmaking process is helpful to reduce the cost in iron and steel production. In this research, the converting process of a 60t converter from a steelmaking plant was studied, by which the calculation and analysis on slag quantity was carried out. Consequently, the policy of decreasing the amount of caustic calcined dolomite while increasing the amount of lime during the early stage of blowing was formulated. It is indicated in the results that the content of magnesium oxide was significantly decreased when the basicity of slag increased by 42.28% in the early stage. Besides, the dephosphorization rate was increased by 1.16 times while the slag quantity was decreased by 12.74%.