In this paper, the reaction of deep desulphurization by magnesium injection is deeply discuss on theoretic. The principle of resulfurization in hot metal is explained. And some process parameters are optimized in practice produce. Some control methods are put forward, e.g. reduce sulfur content in hot metal at the end of injection, deslag the desulfurization slag. By these ways the effect of desulfurization is improved. The resulfurization content in BOF is lower than 10×10^-6. The batch production of antacid pipeline steel is successed.