This paper mainly studies the weight, composition, sulfur content of the desulfurization slag from 150 t ladle after composite injection desulfurization in Meishan Steel, as well as how the sulfur content in hot metal changes before and after desulfurization. Consequently it further studies the range of resulfurization caused by residue slag at the top of the ladle. The results show that after desulfurization the sulfur content of desulfurization slag is from 2% to 3.5%, after slag skimming the weight of residue slag is about 100 kg, and the rate of resulfurization is less than 0.003% in hot metal. Thus, the desulfurization slag is not the main reason leading to resulfurization. Meanwhile, in order to reduce the rate of resulfurization, it can add slag conglomeration agent into the desulfurization slag to improve the effect of slag skimming and reduce the amount of residue slag, iron loss also can be decreased.