Abstract: Since it is hard to get deep desulfurization of hot metal just though pre-treatment process,related processes such as hot-metal pre-treatment,converter steel-making and off-furnace refining are optimized based on the theories of both thermodynamics and dynamics concerning desulfurization.Relevant technological measures are taken to properly and quantitatively control desulfurization level in process steps like hot metal pre-treatment with compound desulfurizer,sulfur content control in converter melting,desulfurizing in tapping and dual-slag deep desulfurizing in LF refining,so as to compensate the inefficiency of deep desulfurization during hot metal pre-treatment,and reduce the desulfurization intensity of the LF process.As a result,it is successful to produce the ultra-low sulfur steel for linepipe to be used under hi-acid service condition,which is in possession of w(S) ≤0.001%,Ca/S≥2.0 and inclusion ratings of A,B,C and D not higher than 0.5.