Calcium slag (CS) is a by-product generated during smelting to extract calcium metal from the ore by aluminothermic reduction method. The melting point, melting speed, viscosities, surface tensions and the refining effects of CS and normal premelted refining slag (NPRS) were measured in the laboratory. Then the plant trials were done in a 150 tons Ladle Furnace (LF) in a steelmaking plant during the refining of clean steel. The results show that the properties of CS are similar to those of NPRS, and CS is a good refining agent not only in laboratory experiment but also in industrial production. By using CS, the average final sulfur content in X70 pipeline steel reaches 25×10-6; the total oxygen content in GCr15 bearing steel is 5×10-6~8×10-6; the titanium content in GCr15 bearing steel is less than 20×10-6 and lower than the steels refined by using NPRS.