The invention discloses a smelting method for modifying sulfide inclusions in non-quenched and tempered steel or pinion steel. A modification process is mainly realized by the following step: feedinga silicon-magnesium-calcium composite core-spun yarn into refined molten steel, or respectively feeding a silicon-magnesium core-spun yarn and a silicon-calcium core-spun yarn into the refined moltensteel, so that the content of magnesium in the steel is 4-25ppm, and the content of calcium in the steel is 5-25ppm. According to the method, by optimizing the contents of oxygen, sulfur, manganese, calcium, magnesium and aluminum in the steel in a refining process, the sulfide inclusions in the non-quenched and tempered steel and the pinion steel are modified effectively, so that the form, size and distribution of sulfide are controlled, the inclusions in the non-quenched and tempered steel and the pinion steel are transformed into 'internal hard oxide particle and external covered sulfide' composite inclusions, and the quality of special steel is enhanced.