The invention relates to a wastewater purifying agent of natural waters. The purifying agent comprises the following components in percentage by weight: 30-70% of benzalkonium chloride, 10-20% of polymeric ferric sulfate, 10-15% of polyacrylamide, 5-8% of activated carbon, 3-5% of an effervescent disintegrant, 20-30% of silica, and 3-5% of aluminum oxide. The content of benzalkonium chloride is 50%. With the adoption of the wastewater purifying agent provided by the invention, the wastewater can be clarified in a few seconds, the wastewater pollution to the environment is reduced, the production cost is lowered, and the pure water supplement is reduced; the wastewater purifying agent is easy to decompose, less in use amount, free of residue and secondary pollution, and simple and convenient to operate, the time for purifying the water quality is short, bacteria can be eliminated effectively and algas can be inhibited effectively, and organic matters, stagnated and floating flocs and the like can be decomposed, and chemical oxygen demand, total nitrogen, total phosphorus can be effectively removed.