To investigate the effect of the quantity of limestone added to and the way of limestone added into the furnace on dephosphorization efficiency, thermal experiment is carried out in a vacuum induction furnace by changing the quantity of added limestone. The experiment results show that it is feasible to substitute limestone for lime to slag, and how the limestone is added to the furnace plays a significant influence on dephosphorization. When the substitution ratio is 40%, thermal experiment gets the best result of dephosphorization efficiency with the final phosphorus content reaching 0.012% from the initial number of 0.12%, and the dephosphorization rate increasing by 24.17%. However, much of the slag is in incrustation state, thus the experiment couldn’t be carried out normally when the ratio reaches 80%. Furthermore, the optimum scheme for dephosphorization with limestone can be described as follows: steelmaking with the substitution ratio of 60% is the best one when lowering production cost is the first concern, while 40% is the best when the quality is cared most.