The invention discloses a water quality purifying agent. A formula of the water quality purifying agent comprises the following components in parts by weight: 15 to 55 parts of potassium hydrogen persulfate composite salt, 0 to 5 parts of sulfamic acid, 5 to 20 parts of citric acid, 1 to 12 parts of sodium carbonate, 0 to 10 parts of sodium bicarbonate, 0 to 0.5 part of magnesium carbonate, 0 to 9 parts of sodium hexametaphosphate, 15 to 30 parts of calcium peroxide, 0 to 1 part of sodium dodecyl benzene sulfonate, 0 to 2 parts of lauryl sodium sulfate, 0.05 to 0.1 part of polyethylene glycol, 5 to 15 parts of anhydrous sodium sulphate and 15 to 25 parts of composite microbial agent. In the formula of the water quality purifying agent, a composite potassium hydrogen persulfate product detoxifying technology and a sewage treatment technology beneficial for microorganisms to restore the micro-ecological balance are used for substituting the existing product capable of leading to the secondary pollution, the advantages such as fast onset of action and lasting effect can be realized, and the water quality purifying agent can take effect within half an hour; the water modification, detoxifying, bacteriostat and water purifying effect can last for 7 to 10 days, and the treatment on the water pollution and the restoration of the ecological balance can be realized in 7 days to 1 month.