The invention discloses a water purifying agent for advanced treatment of pulping, papermaking, printing and dyeing wastewater as well as a preparation method of the water purifying agent. According to the preparation method, the water purifying agent is prepared through a synthetic reaction of iron salt, diatomite, macromolecular polymers and appropriate conditioners. The water purifying agent is mainly characterized in that the content of iron oxide by weight is 8-14%, the solid content is larger than or equal to 30%, and the pH is in a range of 2-3. The preparation method has the characteristics that the operation is convenient, the production energy consumption is low, the emission of three wastes in the process is avoided and the like. The water purifying agent has the characteristics of small dosage, good COD (chemical oxygen demand) reducing and decolorizing effects and the like.