Abstract: Calcium treatment experiments were carried out on L360QS pipeline steel. The effect of calcium and sulfur contents in molten steel on the inclusions modification was investigated. The result shows that sulfur content in molten steel is decreased to the range of 0.002% to 0.003% and the highest desulfurization rate reaches 85.0%,which achieves the expected result. With the sulfur content increasing,the average CaS + MnS content in inclusions increases in liquid steel,however,the CaS content don' t. The effect of calcium content in liquid steel on inclusions modification is greatly obvious,and almost all inclusions in the pipeline steel with high calcium content are modified into spherical calcium aluminate complex inclusions,in which the CaS is uniformly dispersed,and there are few MnS inclusion and MgO·Al₂O₃ inclusions. In order to modify both oxide and sulphide inlusions completely,the sulfur content in molten steel should be controlled about 0.002%,and the calcium content in molten steel should be in the range of 0.003 5% to 0.004 0%,as well as the Ca/S should be above 1.9.