Abstract: The effect of remaining slag operation (remaining slag fraction 1/3 ~ 2/3 slag amount) of BOF steelmaking steel SCM435 on lime adding amount, remaining slag heats of equilibrium basicity and end slag basicity is tested and studied, and the points for attention in slag remaining operation are summarized. As the 80t BOF remaining slag operation is put into practice, the remaining slag rate is up to 68. 9% , the ratio of P content ≤ 0. 015% in product increases from original 49. 3% to 62. 2% , and the lime consumption decreases by 4. 5 kg/t, and the iron loss in slag decreases by 230 kg.