The invention relates to a sewage purification agent, in particular to a sewage purification agent for treating sewage containing phenol, heavy metal, suspended matters, radioactive substances and other contaminations and a preparation process thereof. The sewage purification agent is prepared from the following materials by weight percent; 85-95% of steel slag and 5-15% of polymeric ferric sulfate, wherein the finished agent is in the form of particles, the particle size of the steel slag is no bigger than 80 meshes, and the particle size of the polymeric ferric sulfate is no bigger than 100 meshes. The preparation process comprises the following steps: hydrolyzing the steel slag, drying, stoving, pulverizing, screening, pulverizing the polymeric ferric sulfate, screening, proportioning and compounding. The invention not only provides a new method for treating metallurgical wastes, but also creates a new process for producing the sewage purification agent. The sewage purification agent has multiple functions of adsorption, flocculation, filtration and decoloration, is especially applicable to treatment and purification of sewage containing phenol, heavy metal, suspended matters, radioactive substances and other contaminations, and has the advantages of simple production process, sufficient material resources and low production cost.