The blast furnace slag quenched by iron tailings as modifier was used to prepare slag fiber with the equipment of four-roll centrifuge. The effect of composition on fiber-forming was analyzed, while the trend and mechanism of fiber properties was examined at different acidity coefficient (Mk) and roller speed. The result shows that this method is proper to make slag fiber with the constant roll speed, maintaining the Mk between 1.1~1.3 and the temperature of 1350~1450℃, the physical and chemical indicators of slag fiber meet the national standards.