The invention discloses a sewage purifying agent which is prepared from 70-90 parts of light shale powder and 10-30 parts of a flocculating agent, wherein the light shale powder is prepared by crushing light shale ores, and grinding and separating the light shale ores, wherein roasting is optionally carried out after crushing. The sewage purifying agent provided by the invention can effectively remove alga pollutants, harmful substances, heavy metals or other impurities in the sewage, and can purify the sewage into transparent and clear water free of any pollution; the sewage purifying agent can be repeatedly used; and moreover, the raw materials are easily available, the cost is low, the preparation method is simple, and the large-scale production application is easy to realize.