The present invention belongs to the technical field of environmental materials, and discloses a modified slag adsorbent for anion dye wastewater treatment, and a preparation method thereof. The preparation method comprises: washing, drying, crushing and screening slag; preparing a sodium chloride solution, and mixing the sodium chloride solution and hydrochloric acid to obtain a mixed solution; and slowly adding the mixed solution to the slag, carrying out a stirring reaction, standing, drying, and grinding to obtain the modified slag adsorbent. According to the present invention, the preparation method has characteristics of simpleness, rich raw material source, low price, and waste recycling; after the modification, the micro-pores of the slag are increased, the specific surface area is increased, and the adsorption capacity of the adsorbent is further enhanced; and the adsorbent of the present invention has the good anion dye wastewater treatment effect, has the high equilibrium adsorption capacity, and especially has the good Acid Green 27 adsorption effect.