Slag fiber was prepared with centrifugal spinning apparatus by adjusting the acidity coefficient of molten blast furnace slag with iron ore tailing. The results show that with the increase of acidity coefficient, the diameter, slag ball content and moisture content of slag fiber increase. When the acidity coefficient is 1.4, the fiber diameter increases significantly, being over standard range. When the acidity coefficient is between 1.0 and 1.25, and the content of slag ball is lower, the quality of fiber is better. When the acidity coefficient is between 1.15 and 1.3, the moisture content of fiber changes little and remains at about 0.66%, its tensile strength and toughness are better. In adjusting the viscosity of quenched slag for preparation of the fiber at appropriate acidity coefficient, the coefficient should be controlled between 1.0 and 1.4 on the premise of fiber forming conditions.